Philosophical Review

Form, Substance, and Mechanism

Author(s): Robert Pasnau
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Source: The Philosophical Review, Vol. 113, No. 1 (Jan., 2004), pp. 31-88
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 ThePhilosophicalReview,Vol. 113, No. 1 (January 2004)

Form, Substance, and Mechanism

Robert Pasnau

Philosophers today have largely given up on the project of categorizing

being. Aristotle's ten categories now strike us as quaint, and no attempt
to improve on that effort meets with much interest. Still, no one sup-
poses that reality is smoothly distributed over space. The world at large
comes in chunks, and there remains a widespread intuition, even
among philosophers, that some of these chunks have a special sort of
unity and persistence. These, we tend to suppose, are most truly agents
and subjects, and are what exist in the most proper sense of the term.
We believe, in other words, in substances.
Today these intuitions rest on the slightest theoretical support. The
two dominant theories of substance, the bundle theory and the substra-
tum model, have shortcomings so obvious and well known that philos-
ophers might be excused for supposing that substance talk cannot be
put on any respectable ground.1 There is, however, a neglected philo-
sophical tradition that attempts to account for the distinctive unity of
substances in terms of a single unifying form. This is the much-scorned
scholastic conception of a substantial form, which medieval and
Renaissance philosophers invoked to account for why some segments
of reality have a distinctive kind of coherence and hence a special meta-
physical status.
The modern attitude toward substantial forms is familiar enough.
Descartes dismissively remarked that they are "a philosophical being
unknown to me" (AT 2:367; CSMK, 122). Henry Oldenburg congratu-
lated Robert Boyle on having "driven out that drivel of substantial
forms" which "has stopped the progress of true philosophy, and made
the best of scholars not more knowing as to the nature of particular
bodies than the meanest ploughmen" (3.67). Very soon, 'substantial
form' became a byword for all that was obscure and obsolete in scho-
lastic Aristotelianism, and from this scorn the theory has never recov-
ered. But what exactly were substantial forms? Were they rejected for
good reasons? These are questions that have never been satisfactorily
answered.
The notion of a substantial form has its roots in Aristotle's physical
conception of form as one of the four causes, along with his metaphys-
ical conclusion that form, above all else, is substance in the primary

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sense.2 But this conception of form as somehow substantialtook on new

life among scholastic Aristotelians, and was developed in ways that Aris-
totle himself never suggested. Indeed, I will argue that scholastic phi-
losophers transformed the notion of what a form is, replacing what was
for Aristotle a mode of functional explanation with something much
more like an internal efficient cause. On this account, the road from
Aristotelian function to modern mechanism runs through the medi-
eval understanding of form.
I proceed as follows. After briefly surveying some standard and fairly
unhelpful scholastic formulations (section 1), I suggest that the doc-
trine of substantial form is two-sided, at times appearing concrete and
causal (section 2), and at other times abstract and metaphysical (sec-
tion 3). Both sides of the theory, I argue, serve to explain the special
sort of unity possessed by substances, but in later medieval thought the
concrete side seems ascendant. I then turn to the seventeenth century,
first considering several gross misunderstandings of the theory (sec-
tion 4), and then evaluating the extent to which substantial forms can
be seen to have survived in the work of Descartes, Boyle, and Locke.
Contrary to some recent suggestions, Descartes accepts virtually noth-
ing of the doctrine (section 5). Boyle and especially Locke, however,
can be read as accepting large portions of the doctrine, albeit within a
mechanistic framework (section 6).

1. The Scholastic Doctrine: A First Approximation

From the start, it should be said that if modern attacks on substantial
forms were unjust, the fault lies largely with the scholastics themselves.
Because substantial forms were not challenged within the Aristotelian
tradition, they were not defended or explained in any detail until the
Renaissance. No consensus ever developed about what substantial
forms were, and not even the most articulate of Aristotelians, medieval
or Renaissance, explained the theory very clearly. Even if we put aside
for now the perplexing question of what a form is, there were many dif-
ferent ways in which authors attempted to distinguish substantial and
accidental forms. One proposal, sometimes ascribed to Averroes, was
that substantial forms have as their subject (or inform)something that
only potentially exists, whereas accidental forms have as their subject
something actually existent. This is to say that substantial forms inhere
in prime matter, whereas accidents inhere in a form-matter compos-
ite.3 On another conception of the distinction, associated with Thomas

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 FORM, SUBSTANCE,AND MECHANISM

Aquinas, a substantial form makes a thing be absolutely (simpliciter),

whereas an accidental form makes a thing be in some respect (secun-
dum quid).4Among later Aristotelians, it was standard to characterize
the substantial form in still another way: as that which combines with
matter to make a composite that is unum per se.5The sixteenth-century
Paduan scholastic, Jacob Zabarella, proposed yet another criterion:
that accidental forms cannot be received in prime matter unless a sub-
stantial form has already been received, whereas a substantial form can
be received prior to any accidental forms.6
Eventually, we will be in a position to make some sense of these var-
ious proposals. On its face, however, all of this looks rather discourag-
ing. Even if one or more of these accounts were correct in fixing the
extension of the concepts in question, they seem far from perspicuous
as an explanationof what substantial forms are. Indeed, by the end of
the thirteenth century, John Duns Scotus had concluded that there is
no informative account of how substantial and accidental forms differ.
In practice, according to Scotus, philosophers give various derivative
("a posteriori") accounts of what the difference is: they point to the
having of contraries, to the taking on of more or less, to being known
in its own right, etc. These are all characterizations of accidental forms
and not substantial ones. Still, they don't tell us about the thing in
itself. Itjust is true that pale is an accident, or that humanityis a substan-
tial form. Such propositions are known per se, and in these cases there
is nothing more to be said, because nothing more can be said. For Sco-
tus, the distinction between substantial and accidental forms is basic
and unanalyzable.7
Yet despite the bewildering variety of scholastic accounts, and
despite Scotus's insistence that the concept is primitive, one funda-
mental feature of substantial forms was universally accepted by the
scholastics: that the substantial form plays a part in the essence of a
thing. The classic text is Aristotle, Physics2.3, 194b27, which character-
izes the formal cause as "the account of the essence." Averroes, too,just
before offering the distinction mentioned earlier between substantial
and accidental forms, remarks that the substantial form is what gives a
thing "its name and definition" (Phys. 1.63). And according to Fran-
cisco Sudirez,the end of the substantial form is "to constitute and com-
plete the essence of a natural being" (DM 15.1.18).
For the scholastics, this often went without saying, and was no doubt
regarded as insufficient for any serious analysis of substantial forms.
Scotus, in particular, would not have considered such an account very

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revealing. But this is surely the place where a reconstruction of the

medieval doctrine should begin. It immediately raises the question,
however, ofjust what it means to say,as Sudirezdoes, that the substantial
form "constitutes" a thing's essence. The most straightforward way to
think about this is to suppose that the substantial form of a thing just is
its essence or the set of its essential properties.8 This is not to identify
the substantial form with all a thing's necessaryproperties, because for
an Aristotelian not all necessary properties are essential. The essential
properties are those that define a thing as what it is. (To take the most
hackneyed example, rational is included, whereas risibleis not.) Of
course, this notion of an essential property is itself badly in need of clar-
ification and defense, but that is an issue that will have to wait (for sec-
tion 3), because this initial straightforward proposal faces more
immediate difficulties. First, it cannot be right to identifythe substantial
form with a thing's essence, because there was general agreement
among the scholastics that the essence of a thing includes both its sub-
stantial form and its common matter. It is part of the essence of a
human being, for instance, to be a composite of soul with flesh and
bones.9 Aristotle might be thought to reflect this point, inasmuch as he
describes the form not as the essence, but as the account (logos, ratio)
of the essence. Glossing Aristotle's remark, the Coimbrans make it
clear that the relationship between substantial form and essence is not
that of identity: instead, the substantial form is "that in which the nat-
ural essence of any composite is principallycontained, or what completes
the essence of a thing and its definition, and distinguishes it from oth-
ers" (Physics1.9.10.1).
Here one might want to explore in more detail the notion of com-
mon matter-not thisflesh and thesebones, but flesh and bones in gen-
eral-as something that is part of a thing's essence but lies outside its
form. One might well suppose, contrary to the usual medieval view,
that to pick out the commonmatter of a thingjust is to pick out the for-
mal aspect of the matter, and hence not to have moved outside of form
at all. But I want to set this issue to one side, and focus on a second rea-
son why it is at least misleading to identify a thing's substantial form
with its essential properties. For scholastic philosophers of all persua-
sions, the substantial form is the explanatory basis of the entire sub-
stance, serving as the internal cause of a thing's accidental properties
and supplying the identity conditions for the whole substance and its
parts. These are the causal and metaphysical frameworks that ground
medieval theories of substance. To describe the substantial form

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 FORM,SUBSTANCE,AND MECHANISM

merely as the essence of a thing implies that the scholastics simply pick
out one or more properties of the thing as somehow distinctive or
definitive, and call that set of properties a form. There is, I now want to
suggest, a much more interesting theory here than that, one that
explains substantial unity and identity over time, and at the same time
helps to justify Aristotelian realism with regard to essences.

2. The Concrete Side of Substantial Form

Among the familiar four Aristotelian causes, we can distinguish

between the pair that works internally, form and matter, and the pair
that is external, the efficient and the final cause. It is commonplace to
remark that these four causes are not really causes in our modern sense
of the term, but rather principles of explanation. The internal causes
in particular seem to pick out nothing more than assorted properties
of a substance, and so to offer a very different kind of explanation from
the efficient cause, the only cause on the list that seems properly causal
at all. But matters are more complicated, at least with respect to the for-
mal cause, at least as it was understood by scholastic authors. According
to these Aristotelians, the various forms of a substance are held
together in a tight causal structure, with one form-the substantial
form-producing and sustaining the various accidental forms that give
a substance its particular appearances and qualities. As we will see, the
substantial form can be viewed as playing something very much like the
role of an internal efficient cause, sustaining and regulating the exist-
ence of that which the efficient cause originally produced.
Descriptions of this internal causal framework appear as early as Avi-
cenna (980-1037). He claims that "among accidents, there are some
that occur from without and some that occur from the substance of the
thing." As examples of the latter, Avicenna offer skin color, height, and
the disposition to be hopeful or cheerful.10 By the middle of the thir-
teenth century, Latin authors were routinely ascribing this sort of role
to substantial form. According to Albert the Great, "there is no reason
why the matter in any natural thing should be stable in its nature, if it
is not completed by a substantial form. But we see that silver is stable,
and tin, and likewise other metals. Therefore they will seem to be per-
fected by substantial forms" (De mineralibus3.1.7 [Wyckoff, 173]). To
be "stable in its nature" is for a thing to have a constant set of properties
that are characteristic of that thing. The substantial form is not that set,
but something further that explains their enduring presence. Aquinas

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regularly describes substantial form in a similar way.In his early treatise

De ente et essentia, he remarks that "substance ... must be the cause of its
accidents" (De ente 6.54-57), and though Avicenna is not expressly
mentioned, Aquinas makes use of one of his examples: the black skin
of an Ethiopian. More generally, he later writes, "all accidents are cer-
tain forms added onto the substance, caused by the principles of the
substance" (SCG 4.14.3508).11
Can Aquinas really mean-as some of the textsjust quoted suggest-
that all of a thing's accidents are a determinate product of the substan-
tial form alone? No, he cannot really mean that, because some acci-
dents, like the cut on my left knee, clearly have an external cause. Such
accidents surely cannot be the sole product of my essence. Aquinas
explicitly addresses this issue in various places. He remarks, for
instance, that not every perfection that might come to a thing comes
from its substantial form; instead, "much is added on by supervening
accidents-in a human being, for instance, shape and color are added"
(ST la2ae 18.3c). Aquinas attempts to set out his view with some preci-
sion in the following passage:
There are three kindsof accidents. (1) Some are causedby the principles
of the species and are called propria, as risibleis for a human being.
(2) Othersare caused by the principlesof the individual,and this in two
ways.(2a) Some have a permanent cause in their subject,and these are
inseparableaccidents, such as maleand femaleand others of this sort.
(2b) Othershavea cause in their subjectthat is not permanent,and these
are separableaccidents,such as sittingandwalking.(QDA12 ad 7)
Cuts and scrapes fall into class (2b), inasmuch as they "have a cause in
their subject that is not permanent," like my sitting or walking. Two
other classes of accidents have an internal cause; I will refer to these as
internalaccidents.There are (1) propria: species-wide accidents that are
not part of a thing's essence but are necessary attributes. These flow
from the "principles of the species," which is to say that they flow from
the essence as it is common to all members of a species. Then (2a)
there is the class of accidents that do not necessarily belong to every
member of the species, but that do flow from "the principles of the
individual." In using this phrase, Aquinas means to refer to the partic-
ular form that distinguishes me as an individual. It is a consequence of
my particular form that I am male, blue-eyed, gangly, etc.12
It may be surprising to find Aquinas committed to differences
between substantial forms within a single species. But this is something
he is quite clear about. The main reason why some people are more

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intelligent than others, for instance, is because of differences in their

souls. In reply to an objection that two members of the same species
cannot differ in soul, since that would be a difference in form, which
would make them members of different species, Aquinas replies that
not all differences in form do make for a difference in species (ST la
85.7 ad 3). Given this view, it follows naturally that knowledge of a
thing's individual substantial form would yield knowledge of all its
intrinsic accidents. Aquinas makes this point in a revealing passage
from his De veritate.First, he says, knowledge of the essence of a species
would reveal knowledge of all the accidents associated with that spe-
cies: "an intellect cognizing the essence of a species comprehends
through that essence all of the per se accidents belonging to the spe-
cies" (QDV2.7c). Then, he adds, knowledge of an individual's "proper
essence" would reveal not only the species-wide accidents but even the
individual's peculiar intrinsic accidents, such as skin color and size:
"once the proper essence of a singular is cognized, all of its singular
accidents are cognized" (QDV2.7c).13 Aquinas goes on to say that only
God could have such knowledge of an individual's distinctive nature.
But the metaphysical point remains: all of a substance's properties,
necessary and contingent, either flow from its substantial form or are
imposed from without: "everything that holds true of something is
either caused by the principles of its nature, as is a human being's
capacity for laughter, or comes to it from an external principle, as light
in the air comes from the sun's influence" (De ente4.127-30).
The role of substantial form becomes even more prominent in later
authors. William Ockham, for instance, remarks that "it is clear to the
senses that hot water, if left to its own nature, reverts to coldness; this
coldness cannot be caused by anything other than the substantial form
of the water" (Quodlibet3.6; 226). Later in the fourteenth century,John
Buridan remarks that "substantialforms, rather than the accidents con-
joined to them, are the principal active principles in the changes and
rests to which the forms are suited" (QPhys.2.5; f. 33rb). He illustrates
the causal role played by the substantial form as follows: "When, in
someone with a fever, the heat exceeds its correct proportion to other
qualities, it is not apparent how it would be reduced to its [correct]
state unless the soul were to reduce it" (ibid.).
Renaissance authors, most notably Suairezin MetaphysicalDisputation
15, developed this claim in detail. Near the end of what must be the
most detailed treatment of the topic ever attempted,14 Suairezwrites
that "the most powerful arguments establishing substantial forms are

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based on the necessity, for the perfect constitution of a natural being,

that all the faculties and operations of that being are rooted in one
essential principle" (DM 15.10.61). Sudirezrefers the reader back to an
earlier discussion, where he had argued:
The aggregationof multiplefacultiesor accidentalformsin a simple sub-
stantialsubjectis not enough for the constitutionof a naturalthing.... A
form is required that, as it were, rules over all those faculties and acci-
dents, and is the sourceof all actionsand naturalmotions of such a being,
and in which the whole varietyof accidentsand powershas its root and
unity. (DM15.1.7)
With this, Suirez rejects the bundle theory, according to which various
faculties and forms constitute a substance simply by being collected
together at the same time and place. But he also rejects a substratum
model, on which faculties and forms might make a substance in virtue
of having some kind of underlying subject. Neither model will work,
because there has to be something that, rather than lying beneath
these attributes, "rules over" them, supplying the unity necessary for a
genuine substance.
Su~rez's most detailed set of arguments for this conclusion rests on
the way that substances have natural states to which they gravitate:
water, for instance, is naturally cold, and eventually reverts to that state
even after being heated. What is the cause of this? It must be an inter-
nal principle, Suirez argues, and can be nothing other than a substan-
tial form (DM 15.1.8).15 This is the same example that Ockham had
used before, and that Boyle would explicitly attack in the seventeenth
century (OFQ 60). The governing assumption behind the example is
that substantial forms play a concrete, causal role in regulating the acci-
dental properties of substances.
The Coimbran commentators, contemporaries of Suirez, take
much the same line. They describe how "certain proper and peculiar
functions apply to individual natural things: reasoning to a human
being, whinnying to a horse, heating to fire, and so on in other cases"
(Physics1.9.9.2). This is obviously not a list of essential properties, in
the Aristotelian sense-no one would suppose that whinnying is what
makes a horse be a horse. But still "the origin of such accidents must be
ascribed to the substantial form, as to their source" (ibid.). Summariz-
ing their view about the role of such forms, they write,
In all it cannot be denied that,for each and everynaturalthing, there is a
substantialform, by which it is established,through which its degrees of
excellence and perfection among physical composites is selected, on

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 FORM, SUBSTANCE,AND MECHANISM

which every propagation of things depends, from which its aspect and
character is stamped on each thing, which undertakes whatever task there
is in nature given its power, which elicits all actions both of life and of all
other functions, to which support accidents come, as if instruments, and
finally, which marvelously distinguishes and furnishes the theater of this
admirable world in its variety and beauty. (Ibid.)
This elaborate paean to the substantial form is simply the culmination
of a view that was prevalent throughout the medieval scholastic era.
In all these texts, the dominant conception of form is decidedly con-
crete rather than metaphysical. Substantial forms are understood as
causal agents that would figure centrally in any complete scientific
account of the natural world. They explain why water is cold, why gold
is heavy, why horses have four legs and human beings two, and why
horses merely whinny whereas human beings talk. Given this concep-
tion of form, it is no wonder that some scholastic authors contemplated
describing the substantial form as a kind of efficient cause. Thus Henry
of Ghent contended in the late 1280s that "every subject through its
form is the active and efficient cause of its proper accidents and like-
wise of its common accidents, together with the initial active causes that
concur with it, disposing it for this in the way described above" (Quod.
10.9; 426ri). Henry doesn't hesitate here to describe the substantial
form as an efficient cause, treating it as the internal analogue to the tra-
ditional efficient cause that comes from without. Godfrey of Fontaines,
a contemporary critic, took issue with that characterization, and
insisted that only the initial external causes can be referred to as effi-
cient causes (Godfrey, Quod. 8.2).16 Judging from later authors, God-
frey's view won out as a terminological matter. But the point does seem
to be wholly terminological, inasmuch as the later medieval concep-
tion of substantial form came to have more and more in common with
an Aristotelian efficient cause.

3. The Metaphysical Side of Substantial Form

Looked at from what I am calling their concrete side, substantial forms
play the role of a cause in our modern sense of the term, serving as
something like an internal efficient cause that sustains and regulates
the substance once it has been brought into existence by an external
efficient cause. The ascension of this conception of form in the later
Middle Ages marks a pronounced historical change in the theory of
form. Although Aristotle's conception of form is notoriously open-
ended, it is clear that he wanted formal explanations to hold at a higher

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level of abstraction than that of material and efficient explanations.

According to MetaphysicsZeta 17, the unity of a substance is a product
not just of its elements, but of some further unifying principle, the
form, that is not itself an element. This argument suggests that the for-
mal cause occupies a different conceptual space from that of material
or efficient causes. Whereas explanation at these latter levels is what we
now think of as yielding a narrowly physicalistic picture of the world,
formal explanation seems to take place at a more abstract, metaphysi-
cal level. What is at issue here are not ground-level facts about why a
body has this or that observable quality, but more refined questions of
unity and individuation, requiring judgments about, for instance, a
thing's modal properties.
The scholastic conception of substantial form, particularly as it
emerges in later authors, looks to be quite different. For these authors,
as we have seen, a substantial form is nothing like a metaphysical
abstraction: it is, instead, a causal power acting in a quasi-efficient man-
ner on the various parts and qualities of the substance. Now there are
places where Aristotle himself might be thought to encourage this con-
ception of form, particularly in his biological works.17One might even
suggest that modern Aristotelians have downplayed this side of Aristo-
tle in favor of a more metaphysical conception of form, lest the theory
seem wedded to an empirically false doctrine akin to scholastic real
qualities or modern vitalism. (In this spirit, Kit Fine remarks that "Aris-
totle seems to have a possible basis for the belief [in individual forms],
namely that forms are real and active principles in the world, which is
denied to any right-minded modern" (1994, 19).) So it may be that in
Aristotle, as in the scholastics, form should be seen as having two sides,
not clearly reconcilable, one of which treats forms as individual causal
powers whereas the other conceives of form abstractly.
Still, it seems hard to deny that Aristotle's more pronounced ten-
dency-at least when engaged in high-level philosophical inquiry-is
to treat form abstractly and metaphysically. This is particularly striking
in those passages that suggest that the form of a substance just is its
function. Aristotle remarks, for instance, that "if the eye were an ani-
mal, sightwould be its soul" (De an. 2.1.412b18). Elsewhere, the form of
a house is being "a covering for bodies and chattels" (Met.
8.2.1043a16).18 This conception of form looks to be a long way from
the efficiently causal side of form. Understood as a function, the sub-
stantial form obviously cannot be conceived of as an internal causal
power. Now, to be sure, these two conceptions of form might still be

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closely connected. Form conceived of as a causal power might be char-

acterized as that which gives rise to a thing's function, and the function
might be seen as the characteristic feature of a substance that deter-
mines its nature. To be an eye, for instance,just is to see.Hence, the loss
of that characteristic function might entail the loss of a thing's defining
form, and so the loss of its identity. Still, there clearly seem to be two
conceptions here of what a form is. And whereas in Aristotle a more
metaphysical conception of form seems to predominate, by the end of
the scholastic era the case for form rests entirely on its causal efficacy
as the source of a substance's various intrinsic properties.
But even though the metaphysical side of substantial form seems
largely absent from later scholastic discussions, it was not entirely
neglected during the Middle Ages. Buridan, for instance, in the course
of attacking a reductive account of the soul's actions to its powers,
remarks of his opponent that "through similar arguments, they could
say the same of substantial form's relation to matter: namely, that mat-
ter disposed in one way is fire, disposed in another way is water, air, or
stone. This was the opinion of Democritus and Melissus and those who
say that all things are substantially one thing" (Quaest. dean. 3.11). Buri-
dan does not choose to challenge the assumption that matter variously
disposed might give rise to all the superficial qualities of various sub-
stances. The stress is instead metaphysical: a purely corpuscular
approach to material things would be unable to account for the indi-
viduation of substances.
Aquinas is especially interesting in this regard, for although he
does-as we have seen-stress the concrete side of substantial form, he
also puts substantial form to metaphysical work, using it to articulate
the identity conditions for a substance and its various parts. Moreover,
he does this by connecting substantial form with functional, teleologi-
cal considerations, suggesting that in these contexts form might be
playing the role of a higher-level, abstract explanatory principle. The
passages that most strikingly display the metaphysical side of form are
those in which Aquinas argues that substantial forms, unlike accidental
forms, give existence to the whole substance and to each of its parts.
"This is clear from the fact that both the whole and the parts take their
species from it, and so when it leaves, neither the whole nor the parts
remain the same in species. For a dead person's eye and flesh are so-
called only equivocally" (SCG 2.72.1484).19 Here, what distinguishes a
substantial form from an accidental form is not that it exercises some
sort of quasi-efficient causality over the whole substance, but that it

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individuates the whole substance and its parts, so that when a part is
separated from the whole, the part becomes something else, distinct
notjust in number but even in kind. The eye is no longer an eye, except
in name only (homonymously, as Aristotle put it).20 What seems at issue
here is not the scientist's question of what gives flesh the various observ-
able qualities it has, but the metaphysician's question of what gives a
substance and its parts their identity over time. The answer is that my
flesh remains flesh-and, implicitly, remains my flesh-for as long as it
is informed by the same substantial form.
Elsewhere, Aquinas describes in more detail the way substantial
form plays this metaphysical role in individuating substances. What is
characteristic of a substantial form, he argues, is that

a substantial form perfects not only the whole, but each part. For since the
whole is made up of parts, a form of the whole that does not give exist-
ence to the individual parts of the body is a form that is a composition and
ordering (e.g., the form of a house) and such a form is accidental. (ST la
76.8c)
Here the metaphysical role of substantial form-as that which perfects
each part of the substance-is made to rest not on facts about how the
observable qualities of things are regulated by the form, but on the pre-
supposition that genuine substances are not a mere "composition and
ordering," like a house. But why should we accept this distinction
between substances and artifacts? The passage immediately continues:

The soul, however, is a substantial form, and so it must be the form and
actuality not only of the whole, but of each part. And for this reason, just
as one does not speak of an animal and a human being once the soul has
left-unless equivocally, in the way we speak of a painted or sculpted ani-
mal-so too for the hand and eye, or flesh and bones, as the Philosopher
says. An indication (signum) of this is that no part of the body has its
proper function once the soul has left, whereas anything that retains its
species retains the operation belonging to that species. (Ibid.)
Here again we encounter Aristotle's principle of homonymy for living
things, and here it is supposed to be a consequence of the way that a
substantial form actualizes the whole substance and each part of that
substance. Given that this consequence does indeed hold, it is easy to
see why Aquinas sees a distinction between genuine substances, unified
by a substantial form, and mere artifacts, unified merely by "composi-
tion and ordering."21 For in the case of a genuine substance, the parts
are radically dependent on the substance for their continued exist-
ence. Take away a piece of flesh, and it becomes something else. This

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 FORM, SUBSTANCE,AND MECHANISM

is not the case for non-substances. Take a brick awayfrom a house, and
it remains a brick. So the substance is unified not because it cannot
exist without its parts, but because its parts cannot exist apart from it.
Yet why should we suppose that a living thing is subject to Aristotle's
principle of homonymy, whereas a house is not? Why not say that a
door, apart from a house (or when off its hinges) is no longer a door?
Aquinas offers some guidance at the end of the above passage, when he
remarks that the parts of a living thing lose their function (opus) apart
from the soul. One might wonder whether this suggestion adequately
explains this metaphysical distinction between eyes and doors. But I
want to set that issue aside and focus on what the remark reveals about
how Aquinas conceives of form. Aquinas, like Aristotle, never addresses
the question of how the abstract and concrete sides of form interrelate,
and so the issue seems wide open to interpretation. There is at least a
hint in these passages, however, that Aquinas's conception of form
leans more toward the concrete side. If form here is to be understood
abstractly,then one would expect it to be some kind of functional prop-
erty-if not identical to function, then somehow characterized in func-
tional terms. The reference to proper function at the end of the last
passage seems initially to make good on that expectation. It is signifi-
cant, however, that function is introduced as a mere "indication"
(signum). For Aquinas, 'signum'is a technical term for a certain kind of
evidential support: it indicates that what is being offered is not the rea-
son why a thing is so, but simply a defeasible piece of empirical evi-
dence.22 According to the present passage, then, loss of function does
not cause or constitute loss of identity, but is merely a sign of it. This
shows that-however we are to understand form here-we should not
think of it as an abstract, functional principle.
This seems notably different from Aristotle's approach, in the pas-
sages on which Aquinas is drawing. In Meteorology 4.12, Aristotle asserts
that "whata thing is is alwaysdetermined by its function: a thing really
is itself when it can perform its function; an eye, for instance, when it
can see" (390a10-11). The most natural reading of this passage takes
function itself to be that which determines what the thing is, and so to
be form-especially given the corresponding claim from the De anima,
quoted earlier, that "if the eye were an animal, sightwould be its soul"
(412b18).23 Aquinas shows less inclination to conceive of form in this
highly abstract way.Though he wants to arrive at the same metaphysical
conclusions regarding the unity and endurance of substances, he
arrives there by conceiving of form as a concrete internal power that

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gives rise to a thing's functions. Form has the metaphysical implica-

tions it does because of its causal role in sustaining and regulating each
and every internal aspect of a substance's physical constitution.
Whether or not Aristotle and Aquinas do in fact differ in this way, it
seems clear that each can be found, at various points, emphasizing dif-
ferent sides of form, sometimes inclining more to the abstract side and
other times more to the concrete. The exegetical question of which
side is ascendant seems very much in need of further study.24 There
are, moreover, important philosophical questions to be asked here.
First, one would like to know whether there is some way of synthesizing
these two sides of the Aristotelian notion of form, or whether instead
they are as incompatible as the labels 'abstract' and 'concrete' tend to
suggest. (In describing form as having two sides,I mean to remain neu-
tral on this issue.) If these two sides cannot be combined, then that
leaves the modern-day Aristotelian with a choice regarding which
direction to pursue, and the task of explaining what ramifications that
choice will have for the larger framework of an Aristotelian account.
For instance, if one goes for the abstract side of form, then what
becomes of souls? If, on the other hand, one goes for the concrete side,
then is the view still defensible?
Returning to historical matters, it seems quite clear that, by the time
of the Renaissance, substantial forms had come to be conceived in
wholly concrete terms. The intricate discussions of the topic in authors
like Su irez and the Coimbrans give little or no scope for the sort of
abstract, functional considerations at work in Aristotle and (some-
times) Aquinas. Perhaps in this these scholastics were bad Aristotelians.
But, for better or worse, this is the conception of form that made its way
into the seventeenth century. Descartes, for one, remarks that substan-
tial forms "were introduced by philosophers solely so that through
them an account could be given of the proper actions of natural things,
of which this form was the principle and base" (to Regius,Jan. 1642; AT
3:506; CSMK, 208). Boyle likewise makes a lengthy attack on the view
that there is "in every natural body such a thing as a substantial form,
from which all its properties and qualities immediately flow" (OFQ67).
And Hume would later report that "the Peripatetic philosophy ...
assigns to each of these species of objects a distinct substantial form,
which it supposes to be the source of all those different qualities they
possess, and to be a new foundation of simplicity and identity to each
particular species" (Treatise1.4.3, 221).

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 FORM, SUBSTANCE,AND MECHANISM

Whether these developments should be seen as good or bad

depends on one's perspective. One effect of this changed conception
of form was to diminish the distinctness and autonomy of formal expla-
nations. It is one of Aristotle's most cherished ideas that material and
efficient explanations must be supplemented by a further level of for-
mal analysis. Scholastic authors seem to be sliding ever farther back
toward the materialism Aristotle sought to refute, as if they could not
resist the temptation to ground formal explanation on material and
efficient causes at a deeper level. In turn, as the scholastic conception
of form grew increasingly remote from its metaphysical roots in Aristo-
tle, it became at the same time increasingly naturalistic. Indeed, sub-
stantial forms might well be viewed as an early step in the development
of scientific essentialism. By associating essences with a definite hypoth-
esis about the causal interrelationships within a substance, the theory
provides clear criteria for distinguishing between what would later be
called real and nominal essences. Although, as we will see shortly, the
scholastics were largely pessimistic about whether we can in fact grasp
substantial forms, the theory provides no reason to be tempted by any
sort of conventionalism regarding essences. If an entity is organized by
the kind of causal structure we have been considering, then the inter-
nal basis of that causal structure can be identified as the form or
essence. If there is no such structure, then the entity is not a substance.
(If no entities are so structured, then either there are no substances, or
else the whole theory is wrong.) In any case, the theory of substantial
form comes out as a well-defined hypothesis about the structure of
material beings.25
Modern theories of form and substance grew up in the shadow of
this distinctively medieval conception of form and, as we will see, they
did not entirely cast off that shadow. But once the doctrine of form lost
its proper place as an alternative to material and efficient modes of
explanation, it became easy for the moderns to ignore the metaphysi-
cal aspects of the Aristotelian scheme. With the solitary exception of
Leibniz, the leading modern authors saw the doctrine of substantial
form as a physical doctrine that might be replaced by an adequate cor-
puscular account of the various qualities of bodies. It never seems to
have occurred to authors like Descartes and Boyle that substantial form
might be something other than a scientific hypothesis about why, for
instance, water is cold and fire is hot. In this they were simply following
the scholastic doctrine as they knew it. That in itself, indeed, is one of

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the clearest indications ofjust how far those late scholastics had strayed
from the original Aristotelian notion of form.

4. Modern Misunderstandings of Substantial Form

Early modern philosophers never refuted the doctrine of substantial

form. Like so many scholastic doctrines, the theory collapsed from
inattention rather than argumentation. Scorned and ignored by anti-
Aristotelians, it was at the same time ineptly defended by late scholas-
tics, many of whom as a matter of principle avoided saying anything
original or controversial in philosophical matters.26
It is safe to say that substantial forms were never refuted, because the
most prominent early modem critics-with the notable exception of
Boyle-never took the theory seriously enough to mount a vigorous
refutation. Convinced of the truth of their mechanistic approach, phi-
losophers like Descartes, Hobbes, and Locke never learned enough
about scholasticism to be in a position to refute it. They simply didn't
think it worth their time.27 Accordingly, it is not surprising that what
these authors say about substantial form is largely mistaken. Rather
than dwell on those mistakes, we should consider whether they suc-
ceeded in articulating an alternative account that dispenses with sub-
stantial form.
Still, we should at least note what was actually said about substantial
form in the seventeenth century, if only to move these unfounded crit-
icisms out of the way. There were two stock complaints: first, that the
notion of substantial form is too obscure or occult to be usefully
employed in philosophy; second, that it is unacceptable to treat forms
as themselves substances. The first complaint was expressed vigorously
by Locke, who spoke of "fruitless Enquiries after substantial Forms,
wholly unintelligible, and whereof we have scarce so much as any
obscure, or confused Conception in general" (3.6.10). Part of what
made this doctrine so pernicious, according to Locke, was the way it led
scholastic authors to suppose they grasped the real essences of things.
Terms of art like animalitas and humanitaswere introduced as if to sig-
nify the real essence of things, all because of "the Doctrine of substan-
tial Forms, and the confidence of mistaken Pretenders to a knowledge
that they had not" (3.8.2).
There are several difficulties with this criticism. First, scholastic
authors were the first to stress that they had no grasp of what substantial
forms actually were. Scotus put this point as starkly as anyone, remark-

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ing that "no substance is understood in its own right, except in the most
universal of concepts, like being."As for the fact that we do seem to offer
definitions of various substances, Scotus dismissively replies that "with
respect to substances we have a vocal disposition,just as someone blind
is naturally able to syllogize about colors" (QMet 2.2-3, 115-19). That
is to say:we have words that we use, but we don't know what we are talk-
ing about. All the other leading scholastics made similar claims.28 Nev-
ertheless, seventeenth-century authors regularly supposed themselves
to be disagreeing with the scholastic doctrine by insisting on the obscu-
rity of substantial forms. Many recent scholars have repeated this mis-
take.29
A second difficulty with this first stock criticism is that it tends to pre-
suppose that substantial forms are useful only as part of a detailed phys-
ical account of the natural world. Now, as I have been stressing, this was
an important strand of scholastic thinking about form, and to that
extent the criticism is apt, especially in the context of the increased
early modern interest in the details of scientific explanation. But when
substantial forms are conceived in more abstract, metaphysical terms,
then it is less clearjust how damning the criticism is. For even if scho-
lastic authors were unable to give a detailed account of the particulars
of a given substantial form, it might still be of vital philosophical impor-
tance to postulate such forms. If, for instance, forms play a crucial role
in the identity conditions of substances, then it may be enough for that
purpose to be able to explain in general terms what such forms are,
even if their nature in particular cases remains obscure. Since modern
authors tended to focus exclusively on the concrete, causal side of
form, they could see no point in the scholastics' admittedly schematic
accounts.
The second stock criticism of substantial form was that it amounted
to postulating forms that are themselves substances. It ought to be
something of a scandal that this criticism became so widespread in the
seventeenth century, because in fact this blatantly misconstrues the
scholastic doctrine. Now it certainly is true that substantial forms were
understood as a real power within a substance, something over and
above the purely corpuscular constitution of a body. No scholastic
would have tolerated a reductive explanation of form in mechanistic
terms, and this explains why the doctrine of substantial form-along
with the doctrine of real qualities-was such a prominent target for
early modern mechanists. Still, to insist that form cannot be explained
in reductive corpuscular terms is not to treat it as a substance, if by that

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one means a subsistent entity, capable of independent existence apart

from matter. In fact, if there is anything that all the scholastics could
agree on regarding substantial forms, it is that they are not-with the
one exception of the rational soul-substances in that sense. Here, for
instance, is Aquinas:

Just as substantial form does not have absolute existence per se without
that which it informs, so neither does that which it informs, the matter.
That existence in which the thing subsists per se is thus a result of the con-
junction of both. (De ente6.24-28)
And here is Eustachius a Sancto Paulo, in a textbook from the early sev-
enteenth century:

For each principle, matter and form, there is the highest necessity
between them, ... because one can never be found without the other-if
you put aside the rational soul. For just as matter can never exist stripped
of form, so neither can form exist placed outside of matter, since one is
for the sake of the other. (Summaphilosophiae,Physica 1.1.2.9; 127).30
Here 'form' means substantialform. To say that it cannot exist outside of
something else just is to say that it is not subsistent, and so not a sub-
stance in the central sense of the term.
This last text is from one of the few scholastic works that we can con-
fidently say Descartes read.31 Nevertheless, Descartes treats substantial
forms as substances, in the sense of being subsistent:

To prevent any ambiguity of expression, it must be observed that when we

deny substantial forms, we mean by the expression a certain substance
joined to matter, composing with it a merely corporeal whole, and which,
no less than or even more than matter-since it is said to be an actuality,
and matter only a potentiality-is a true substance, or a thing subsisting
per se. (to Regius, January 1642; AT 3:502; CSMK,207)
Various proposals have been made for why Descartes would insist on
treating substantial forms as substances, when this so clearly went
against the scholastic consensus.32 But part of the explanation must be
that this was simply a very widespread misconception among nonaca-
demic philosophers in the seventeenth century. Boyle, to take another
example, criticizes substantial forms on the ground that they are "imag-
ine [d] to be a very substance, indeed a kind of soul" (OFQ 38). Against
this, Boyle argues that they are "substances in name but accidents in
truth" (OFQ 57). This, however, is a false dichotomy: the scholastics
conceived of substantial forms as neither substances nor accidents.33

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These misunderstandings did have some basis in fact. The Aristote-

lians did standardly claim that substantial forms are substances; this was
a standard claim for the same reason that so many scholastic dicta
became standard, because Aristotle said it:
We say that one kind of being is substance (ousian).One [sort] of sub-
stance is matter,whichby itself is not a particularthing. Another is shape
or form, which is that by which a particularthing is so-called.A third is
what is [composed] of those. (Dean. 2.1.412a6-9;cf. 414a14)
This and other such passages led later Aristotelians, especially in the
Renaissance, to describe both substantial form and matter as sub-
stances. Suirez offers this definition: "form is a simple and incomplete
substance, which as the actuality of matter constitutes with it the
essence of a composite substance" (DM 15.5.1). Eustachius follows the
same lines, remarking that substantial form is an "incomplete sub-
stance or, so to speak, a semisubstance, which conjoined with matter
constitutes one whole substance" (Summaphilosophiae,Physica 1.1.2.5;
124). It is not at all clear, at first glance, how to understand this notion
of an incomplete substance. If to be a substance requires the capacity
for independent existence-which is to say subsistence--then the idea
of a "semisubstance" seems to make little sense: a thing either will or
will not be a substance. Now one might suppose that these authors have
the following in mind: that substantial forms are freestanding, inde-
pendent entities, and hence substances, but that they are incomplete
in that they do not naturally occur apart from matter. It would be nat-
ural for Descartes in particular to give such passages this reading,
because he tends to think of soul and body as substances in just this
way.34 But the scholastics clearly meant something quite different.
Suirez goes on to warn that 'substance' is predicated analogously of
form, matter, and the composite (DM 15.7.6), referring the reader to
his later disputation on substance. And in that later work, distinguish-
ing between complete and incomplete substances, he explains that "an
incomplete substance is said to be everything that is a part of a sub-
stance or is conceived in the manner of a part, in which way matter and
form are substances" (DM 33.1.5). Matter and form are substances,
then,just in virtue of being parts of substances. This shows just how lit-
tle force Sudirezgives to the claim that substantial forms are incomplete
substances. 35
Many other scholastics likewise explain the sense in which substan-
tial forms are substances (or, explain the force of the adjective 'sub-
stantial'). Zabarella, for instance, remarks that "by taking 'substance'

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in various ways, we can maintain that [a] form is substance above all,
that [b] the compositeis substance above all, and that [c] matteris sub-
stance above all" (De anima 2.3; 116F). The last follows when 'sub-
stance' is taken as substratum.To get (b), 'substance' must be taken as
that which subsists. For (a) to hold, 'substance' must mean essence.
Zabarella refers back to the sixth-century Greek commentator Simpli-
cius, who had remarked of form and matter that "neither are substance
in their own right, but only substantial (oudiode)," inasmuch as they
are the constituents of what is properly a substance (InPhys. 1.6;
201.24-25). This is a remark that the scholastics could have embraced
without exception.

5. Mechanism without Form: Descartes

A thorough study of early modern treatments of substantial form

would have to take account of a wide range of material. In between con-
firmed anti-Aristotelians like Descartes and rigid Aristotelians like the
Coimbrans, there were many complex attempts to mix Aristotelian and
modern principles.36 But it was the anti-Aristotelians who carried the
day, and so any assessment of what happened to substantial form has to
focus largely on those who did the most to discredit the doctrine. In
these last two sections I will pay particular attention to Descartes, Boyle,
and Locke, all of whom heatedly express their opposition to substantial
forms.
What makes the case of Descartes particularly interesting is that he
rejects substantial forms in general, but expressly invokes the doctrine
for the single case of the rational soul. There is, therefore, a question
why Descartes dispensed with the doctrine in general, and a question
why he retained the doctrine in one special case. Regarding the first
question, Descartes adhered in his published writings to a fairly disci-
plined stance regarding substantial forms: he does not positively reject
them, or argue against them, but simply proceeds without them, hop-
ing to show by example that they are unnecessary. When he does men-
tion these and other scholastic doctrines, he generally proceeds
cautiously, as in the following passage from his Meteorology:
But to keep the peace with the philosophers, I have no wish to deny what-
ever they may imagine in bodies over and above what I have described,
such as their "substantial forms," their "real qualities," and the like. But it
seems to me that my arguments will be all the more acceptable in so far as
I can make them depend on fewer things. (Discourse 1; AT 6:239; CSM
1:187 n.)

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In saying that his view depends on "fewer things," Descartes has in

mind his view that there isjust a single kind of matter shared by all bod-
ies, giving rise to different surface phenomena in virtue of differences
in its motion, size, shape, and arrangement.37
Descartes's most extensive and frank remarks on this topic come in
a long letter (from January 1642) advising his then-disciple Regius on
how to deal with attacks on their shared views. Descartes suggests that
Regius make this reply to his principal critic, Voetius:
I wholly agree with the view of the learned Rector that those "harmless
entities" called substantial forms and real qualities should not be rashly
expelled from their ancient territory. Indeed, up to now we have certainly
not rejected them absolutely; we merely claim that we do not need them
in order to explain the causes of natural things. We think, moreover, that
our arguments are to be commended especially on the ground that they
do not in any way depend on uncertain and obscure assumptions of this
sort. Now in such matters, saying that one does not wish to make use of
these entities is almost the same as saying one will not accept them.
Indeed, they are accepted by others only because they are thought neces-
sary to explain the causes of natural effects. So we will be ready enough to
confess that we do wholly reject them. (AT 3:500; CSMK, 207)
Descartes goes on to suggest several arguments that Regius might make
against substantial forms, but this passage illustrates the heart of his
view: substantial forms are not needed, hence should not be made use
of, hence are in effect rejected. Interestingly, Descartes shows himself
aware of the concrete, causal role of substantial forms, when he
remarks here that others embrace them "only because they are
thought necessary to explain the causes of natural effects." It is striking
that Descartes takes this to be the only reason for postulating such
forms. This may be a distortion of Aristotle, but it is faithful (or so I
have been arguing) to how scholastic authors increasingly came to con-
ceive of these matters.
When substantial forms are so conceived, the central question-
indeed, the only question-becomes whether a purely mechanistic
account can explain the "natural effects" without appealing to substan-
tial forms. Later in the same letter, still as part of the reply proposed on
behalf of Regius, Descartes returns to this issue:

They were introduced by philosophers solely so that through them an

account could be given of the proper actions of natural things, of which
this form was the principle and base, as was said in an earlier thesis. But
no account can be given of any natural action through these substantial
forms, since their defenders admit that they are occult and that they do

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not understandthem themselves.For if they say that some action pro-

ceeds from a substantialform, it is the same as if they were to say that it
proceeds from something they do not understand,which explains noth-
ing. (AT3:506;CSMK,208-9)
Rather than appealing to the explanatory power of his own account,
Descartes now shifts his focus to the worthlessness of scholastic
accounts. He here shows himself aware not only of the concrete, causal
side of the theory, but also of the scholastic willingness to concede their
ignorance of what a given thing's substantial form truly is. But this is a
fatal concession, Descartes believes, because an explanation in terms of
something itself unknown is of no value; it "explains nothing." Return-
ing again to the merits of his ("our") own approach, he goes on to
remark that "the essential forms that we explain give manifest and
mathematical accounts of natural actions" (ibid.).38
In this last remark, Descartes is willing at least to tolerate talk of
forms,as long as they are the sort that "we explain." This raises the ques-
tion of whether we might conclude that Descartes still has something
like a theory of substantial forms, albeit a theory couched in mechanis-
tic terms. In the following section I will consider the coherence of com-
bining mechanism with substantial forms. But here there is no reason
to take that possibility seriously, because-apart from the special case
of the rational soul-Descartes's account has very little in common
with that scholastic doctrine. Of course, Descartes is concerned with
explaining the basis of the properties and actions of material sub-
stances. His mechanistic accounts are attractive largely because of their
potential to explain a vast range of surface phenomena-color, sound,
etc., but also behavior of all kinds-in terms of straightforward modes
of extension. But although this was likewise part of what substantial
forms were intended to do, Descartes's mechanistic proposal entirely
lacks what was central to the scholastic account, an explanation of why
some bodies have a special sort of substantial unity that mere aggre-
gates lack.
The result, for better or worse, is that Descartes's ontology of sub-
stances is much more liberal than the scholastic one. Artifacts, for
instance, are substances just as much as living organisms. A man's
clothing counts as a substance,39 and in general Descartes remarks, "I
do not recognize any difference between artifacts and natural bodies
except that the operations of artifacts are for the most part performed
by mechanisms that are large enough to be easily perceivable by the
senses" (Principles4.203). Descartes entirely lacks an account of why

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certain chunks of matter seem to possess a unity (synchronic or diach-

ronic) that others lack. Bodies necessarily have extension, and poten-
tially come in various shapes, sizes, and patterns. But there is nothing
here to account for why a single body endures, or even what makes for
a single body. Bodies cohere, he writes, "from the simple fact that they
are at rest relative to each other." The argument is worth quoting in
full:

We certainly cannot think up any kind of glue that could join together the
particles of two bodies any more firmly than is achieved simply by their
being at rest. For what could that glue be? It could not be a substance, for
since the particles are themselves substances, there is no reason why
another substance should join them more effectively than theyjoin them-
selves together. Nor could the 'glue' be any mode distinct from their
being at rest. For no other mode could be more contrary to the motion
that separates them than their being at rest. And we recognize no other
kind of things apart from substances and their modes. (Principles2.55; cf.
2.63).
This is utterly implausible, but we can see what motivates it. The stric-
tures of Descartes's physics preclude him from allowing that the parts
are attracted to one another by some kind of force. At the same time,
his hostility to scholasticism leaves him unwilling to appeal to some-
thing that would be neither a substance nor a mode, but a substantial
form, defining and regulating the substance's various modes.
Such forms will seem most plausible in the case of living organisms,
and least plausible in the case of artifacts that can be assembled and dis-
assembled at will. No wonder, then, that the scholastics insisted on dis-
tinguishing these cases, whereas Descartes ran them together. (Sticks
and stones will be an uncomfortable middle ground for the scholastics,
for whom it is far from clear that such nonliving aggregates genuinely
are substances.) It is not obvious whose position is more defensible.
The scholastics have a robust explanation of substantial unity, but as a
result they must make some questionable distinctions between what
does and does not count as a substance. Descartes can be much more
ecumenical (if not downright casual) about substancehood, but he can
do so because he isn't constrained by a systematic account. Whether
these questions are all that important depends on just how much
weight one gives to substantial unity. For Aquinas, as for the scholastics
in general, "each and every thing is a being only to the extent that it is
one thing" (QDA 11lc). This may or may not be innocuous. At times,
Aquinas pushes for some fairly radical conclusions:

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but one thing as an

But if you saythat Socratesis not one thing simpliciter,
aggregate of mover and moved, then many absurdities follow. First, since
each and everything is one and a being in the same way,it followsthat
Socrates is not a being, and [second] that he is not in a species or a genus,
and further [third] that he does not have an action, since an action
belongs only to a being. Hence we do not say that a sailor's thinking is the
thinking of the whole that is the sailor and the ship, but of the sailor only.
(De unitate3.148-58)
In general, for Aquinas, only something that is one in the fullest sense
-that is, only a substance-can properly be said to exist in the fullest
sense. Other things, like heaps and aggregates, do exist, but only in an
extended sense of the term. Properly, we would not even say that such
quasi-beings perform actions. Instead, their substantial parts do.
Descartes is not in a position to worry about such matters, even if he
wanted to. As evidence ofjust how far he is from focusing on this aspect
of substantial form, consider how he argues against the presence of a
substantial form-a soul, that is-in nonhuman animals. For the scho-
lastics, the soul of an animal would explain the enduring presence
within the animal of its various defining and accidental properties. Des-
cartes of course thinks that he can explain all this in mechanistic terms.
What need is there, then, for a soul or substantial form? In arguing
against the scholastics on this score, Descartes can see no argument for
animal souls other than their need to perform the various functions of
movement and life that we ascribe to animals.40 His reply therefore
consists in either insisting on the adequacy of a mechanical account, or
else (notoriously) denying animals the capacity to perform these func-
tions. Yet all this obscures the central issue regarding substantial forms.
Of course, an orthodox Aristotelian would dispute the adequacy of
such mechanistic accounts. But the rationale for a substantial form
would be based, above all, on the need to explain an animal's striking
unity and continuity over time. As we have seen, Descartes has nothing
better to say on this score than to make appeal to "the simple fact that
they [bodies] are at rest relative to each other."

Descartes's general attitude toward substantial forms-that they are

simply unnecessary-is nicely calculated to leave room for substantial
forms in the one case where they do have explanatory force: the case of
human beings. In his correspondence with Regius, he writes that the
soul is "the true substantial form of man" (AT 3:505; CSMK, 208).41 To
Arnauld's worry that Meditation 6 turns a human being into a soul
making use of a body, Descartes replies,

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 FORM, SUBSTANCE,AND MECHANISM

I thought I was verycareful to guard againstsomeone's supposing from

this that man is simplya soul using a body. For in the Sixth Meditation,
where I dealt with the distinction between mind and the body, I also
proved at the same time that the mind is substantiallyunited with the
body. (FourthSet of Replies;AT 7:227-28;CSM2:160)
Meditation 6 is indeed full of descriptions of man as a composite of
mind and body, culminating in this explicit remark: "Nature also
teaches, by these sensations of pain, hunger, thirst and so on, that I am
not merely present in my body as a sailor is present in a ship, but that
I am very closely joined and, as it were, intermingled with it, so that I
come together with the body as one thing" (AT 7:81; CSM 2:6). But
Descartes's argument here, which appeals to the mind's feelingdamage
to the body, hardly yields any precise results about howmind and body
are connected. Although Descartes would later claim that they are
"substantially united" (as above), it is unclear to just what extent Des-
cartes is accepting the scholastic doctrine of substantial form.
There has in recent years been considerable discussion of this
issue.42 In light of the foregoing, we can reach a more informed judg-
ment about where Descartes stands. With respect to the concrete side
of the doctrine, Descartes clearly did not adhere to the standard medi-
eval conception of the relationship between substantial form and mat-
ter. On that conception, the substantial form regulates a body's
physical attributes. For Descartes, in contrast, all bodies-even the
human body-can be explained in mechanical terms without recourse
to substantial forms. Thus, he remarks, again to Regius (Dec. 1641),
that "when we consider the body alone we evidently perceive nothing
in it demanding union with the soul" (AT 3:461; CSMK, 200).
Although the mind does of course exert causal influence on the body,
through the pineal gland, it does not define and sustain the body in the
way that a substantial form would.
Against this conclusion, one might invoke Descartes's repeated
claim that the soul, although unextended, informs the whole body: "we
need to recognize that the soul is reallyjoined to the whole body, and
that we cannot properly say that it exists in any one part of the body to
the exclusion of the others" (Passions 1.30).43 This suggests the kind of
dependence, hence unity, intended by the scholastic account. But Des-
cartes immediately goes on, in the next article (1.31), to introduce the
pineal gland: "We need to recognize also that although the soul is
joined to the whole body, nevertheless there is a certain part of the
body where it exercises its functions more particularly than in all the

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others." More decisively, the following article (1.32) concludes that

"apart from this gland, there cannot be any other place in the whole
body where the soul directly exercises its functions." So although in
some sense the mind exists in every part of the body, it has direct causal
efficacy on only one small part, in the middle of the brain. This pre-
cludes the mind from playing the causal role that the scholastics stan-
dardly ascribed to a body's substantial form.
Still, although Descartes clearly has no room for the concrete side of
substantial form, there are suggestions that he embraces the metaphys-
ical side. His insistence that the mind informs each part of the body,
even though it is not causally efficacious on each part, invites a meta-
physical reading: that each part of the body is made what it is by being
united to a rational soul.44 This would be rather speculative if not for
certain comments that Descartes made in correspondence with Denis
Mesland in 1645. When we speak of a man's body, Descartes writes, we
are referring to whatever matter is united with the soul of that man.
"Andso, even though that matter changes, and its quantity increases or
decreases, we still believe that it is the same body, numerically the same
body, so long as it remains joined and substantially united with the
same soul" (AT 4:166; CSMK, 243).45 As we saw in section 3, this is pre-
cisely the metaphysical role that the substantial form of a substance was
thought to play. So even without endorsing the concrete side of sub-
stantial form, Descartes might have appealed to this metaphysical doc-
trine to account for a human being's substantial unity.
But if this is an idea that Descartes toyed with, it is certainly not one
to which he was committed. He never mentioned this doctrine in his
published works, and never showed any signs-in any of his surviving
writings-of wanting to use it to explain mind-body union.46 Indeed,
it might have been dangerous for him to do so, because by giving this
role to substantial form in the human case, Descartes opens himself up
to the possibility that substantial forms might play an individuating role
in all substances-or at least in all living things. For if this is how the
human body is individuated over time, then why not give a similar
account for horses and cows and trees? Here would be the sort of useful
work for substantial form that Descartes was dead set on eliminating. It
is not surprising, then, that his published work is notjust silent but pos-
itively adverse to this kind of metaphysical role for substantial form. In
ThePassions of the Soul, his last work published during his lifetime, he
describes "a very serious error that many have fallen into, and that I
regard as the primary cause of our failure up to now to give a satisfac-

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tory explanation of the passions and of everything else belonging to the

soul."

The error consists in supposing that since all dead bodies are devoid of
heat and movement, it is the absence of the soul that causes this cessation
of movement and heat. Thus it has been believed, without reason, that
our natural heat and all the movements of our bodies depend on the soul;
whereas we ought to hold, on the contrary, that the soul takes its leave
when we die only because this heat ceases and the organs that bring about
bodily movement decay. (Passions 1.5)
This is not incompatible with the metaphysical role proposed in the
correspondence with Mesland. What Descartes here denies is the con-
crete, causal role of substantial form. He might consistently deny this
and continue to hold that, although the mind separates from body
because the body ceases to function, the body ceases to exist when the
mind departs. Yet the very next article extends these causal consider-
ations into the abstract, metaphysical side: "So as to avoid this error, let
us note that death never occurs through the absence of the soul, but
only because one of the principal parts of the body decays" (Passions
1.6). This is still,just barely, compatible with the metaphysical proposal
to Mesland. If we think of the death of the body not as the body's ceas-
ing to exist, but as a physical process of decay, then Descartes can main-
tain that the mind (soul) determines the body's existence over time but
is not responsible for the cessation of life. What he says next, however,
seems impossible to reconcile with the abstract, metaphysical side of
substantial form: he goes on to compare "the difference between the
body of a living man and that of a dead man" to the difference between
a watch that is working and "the same watch or machine" when it is bro-
ken (ibid.). This is not what Descartes should say if he takes seriously
his remarks to Mesland. If a watch stands to working as the human body
stands to being alive, then a broken watch should not be "the same
watch"-not if Descartes accepts that the soul is what gives the body its
identity conditions. Indeed, the very point of this metaphysical concep-
tion of substantial form is to yield a kind of unity not present in the case
of machines assembled by hand. Descartes is quite clearly going out of
his way to dismiss that entire project.
So although Descartes sometimes refers to the mind as the body's
substantial form, there is no reason to think that he is putting this claim
to any serious philosophical use.47 That is not to say that his remarks in
this area were dishonest, or even disingenuous. In the vast majority of
cases, in discussing the relationship of mind and body, Descartes makes

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no appeal at all to the mind's status as a substantial form. On the few

occasions when he does so, he makes no claim that this way of speaking
does any philosophical work. His attitude rather seems to be that it
does no harm-in the case of the human soul-to speak of it as a sub-
stantial form. It seems to me that the following remark accurately
reflects Descartes's true attitude:

Whenever the occasion arises, in private and in public, you should give
out that you believe that a human being is a true ens per se, not an ens per
accidens,and that the mind is really and substantially united to the body.
You should say that they are united not by position or disposition, as you
assert in your last paper-for this too is open to objection and, in my opin-
ion, not true-but by a true mode of union, in the way everyone com-
monly accepts, even if no one explains in what way this is so, and so you
need not do so either. (AT 3:493; CSMK, 206; cf. AT 3:508)

Again this is Descartes's advice to Regius, under external pressure. But

this is not a verbatim description of what Descartes thinks Regius
should say. Here the advice comes with a kind of private gloss indicat-
ing what their attitude toward such terminology should be. What
emerges is that Descartes thinks no one has a clear sense of what it
means to use expressions like 'ens per se', 'really and substantially
united', and 'true mode of union'. Certainly, he implies, it would be
better to avoid such language entirely. But since others throw these
terms around without giving them any well-defined sense, and since
Descartes himself believes that the connection between mind and body
is in fact quite intimate, there is no reason to make trouble for oneself
by rejecting this traditional and authoritative language. In effect, since
he judges that the scholastics themselves don't have a clue about what
these concepts involve, Descartes seems to feel free to invoke such lan-
guage himself. By now, however, it should be clearjust how wrong that
judgment was, and how the lack of substantial form leaves Descartes
awkwardly positioned to account for the unity of substances.48

6. Mechanism with Form: Boyle and Locke

Descartes's aggressively reductive form of mechanism did not go
uncriticized, even by his fellow mechanists. Gassendi, for instance,
challenged Descartes's claim to have distinctly perceived the wax itself
through the mind:
Besides the color, the shape, the fact that it can melt, etc., we conceive
that there is something that is the subject of the accidents and changes we
observe; but what this subject is, or what sort of thing it is, we do not know.

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This alwayseludes us; and it is only a kind of conjecturethat leads us to

think that there must be something underneaththe accidents. (FifthSet
of Objections;AT 7:271;CSM2:189;cf. AT 7:338,AT 7:275)
Descartes took the essences of things to be clearly evident, for anyone
who would use the mind alone to perceive them. His ontology is con-
sequently as transparent and parsimonious as one could want: there
are extended things, in various modes, and there are thinking things,
in various modes. Sometimes-in the human case-those things are
intimately united. For Gassendi, this ontology is incomplete, because it
fails to capture the nature of the subject that underlies the accidents.
What this subject is, we do not know, and perhaps cannot know.
This conjecture of an underlying subject would have been congenial
to friends of the substantial form. Gassendi himself does not move the
claim in that direction, but his line of thought would resonate widely
with later mechanists, some of whom would retain more of that Aristo-
telian framework.49 In this final section I want to show how it was pos-
sible in the seventeenth century to be a mechanist and at the same time
to embrace central aspects of the scholastic theory of substantial form.
This is particularly evident, I will suggest, in John Locke.
It is fairly well known that early modern philosophers largely
retained the notion of form. Kant, to take one particularly striking
example, would remark of matter and form that "these are two con-
cepts that ground all other reflection, so inseparably are they bound up
with every use of the understanding. The former signifies the deter-
minable in general, the latter its determination" (Critiqueof Pure Rea-
son, A266/B322). Seventeenth-century philosophers likewise used the
concept of form in various ways,50even while vigorously rejecting the
notion of a substantialform. This stance has made it easy to suppose
that substantial forms are simply incompatible with genuine mecha-
nism. But that widespread assumption (then and now) is simply not so.
The mistake stems partly from the belief that substantial forms must be
substances, perhaps even immaterial substances (section 4). Beyond
that all-too-common mistake, there is a confusion here between differ-
ent aspects of the theory. As we have seen, one side of substantial form
is metaphysical, and claims in this domain might be made regardless of
whether one's underlying physical theory is Aristotelian or mechanis-
tic.
This should be clear enough from the prominent example of Leib-
niz, who was both a committed mechanist and a proponent of substan-
tial forms. Thus, he contends, on one hand, that

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the consideration of these forms serves no purpose in the details of phys-

ics and must not be used to explain particular phenomena. That is where
the Scholastics failed, as did the physicians of the past who followed their
example, believing that they could account for the properties of bodies by
talking about forms and qualities without taking the trouble to examine
their manner of operation. (Discourseon Metaphysics10)
He then goes on:

This misunderstanding and misuse of forms must not cause us to reject

something whose knowledge is so necessary in metaphysics that, I hold,
without it one cannot properly know the first principles or elevate our
minds sufficiently well to the knowledge of incorporeal natures and the
wonders of God.51
The details of Leibniz's account are too complex and idiosyncratic to
be discussed here. Even so, it is plain that Leibniz fits into our story
quite naturally. As we have seen, scholastic authors defended substan-
tial form by treating it in highly concrete, causal terms. When this
approach came under attack in the seventeenth century, it would be
perfectly natural to look again toward a more metaphysical under-
standing of form-a project Leibniz carried out in his own distinctive
way.
Even as regards the concrete, causal side of substantial form, it is pos-
sible to combine central features of the doctrine with mechanism. In
particular, one might identify some basic form from which all of a
thing's properties flow, and then go on to give a mechanistic interpre-
tation of what that form is and how it generates the properties of a sub-
stance. Of course, a theory of this sort would be more mechanistic than
Aristotelian: gone would be the four elements, real irreducible quali-
ties, and the occultness of substantial form. But though these doctrines
are incompatible with the mechanistic philosophy, substantial form
itself is not.
It is this last kind of view-mechanism combined with a concrete
conception of form-that Boyle shows signs of embracing. Boyle
attacked the theory of substantial form at length, largely because of its
obscurity, and because he took the theory to be committed to forms as
independent substances. But rather than attempting to do away
entirely with the notion of form, as Descartes sometimes seemed
inclined to do, Boyle offered a modified account:

And so, though I shall for brevity's sake retain the word form, yet I would
be understood to mean by it not a real substancedistinct from matter, but
only the matter itself of a natural body, considered with its peculiar man-

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ner of existence, which I think may not inconveniently be called either its
specificalor its denominatingstate, or its essential modification-or, if you
would have me express it in one word, its stamp. (OFQ40)

Boyle takes himself to have modified the scholastic account in two ways.
First, this kind of form or stamp is not a substance-elsewhere Boyle
prefers to speak of it as an essential form52-but the modification of the
matter. Second, Boyle gives a definite interpretation to doctrines that
"are wont to be treated of by scholastic philosophers in so obscure, per-
plexed, and so unsatisfactory a way ... that it is very difficult for any
reader of but an ordinary capacity to understand what they mean"
(OFQ 3). In place of these occult, irreducible forms, Boyle offers defi-
nite mechanical hypotheses regarding the physical properties that give
a body its distinguishing characteristics.53
Boyle may be said to have partly retained the doctrine of substantial
form inasmuch as he preserves something of the concrete, causal side
of the theory. He does not merely identify the form with the essence of
the thing-that alone would not count as retaining the doctrine. His
further claim is that the form shapes the whole body, determining all
of a thing's properties. That is suggested, for instance, in the following
passage:
[T]hat, which is commonly called the form of a concrete, which gives it its
being and denomination, and from whence all its qualities are in the vul-
gar philosophy, by I know not what inexplicable ways, supposed to flow,
may be in some bodies but a modification of the matter they consist of;
whose parts, by being so and so disposed in relation to each other, consti-
tute such a determinate kind of body, endowed with such and such prop-
erties. ("Experiment relating to Salt-Petre," in Works1:237)

Boyle here identifies a disposition or modification from which is con-

stituted "a determinate kind of body endowed with such and such
properties." This disposition is what the previous passage referred to as
the "stamp" and the "form" of the body, and what Boyle more often
refers to as its "texture."54 The properties constituted by this texture
are of course only those that I have been calling intrinsic properties.
Since Boyle thinks that many qualities are not intrinsic in this way but
instead relational (his famous secondary qualities), there is less scope
in Boyle than in the scholastics for ascribing an explanatory role to the
substance's intrinsic form. Still, he is retaining a key scholastic assump-
tion: that there is some one state from which a thing's intrinsic prop-
erties flow. This suggests that he has not completely abandoned the
doctrine of substantial form.

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Can a conception of form as mere motion, size, shape, and posi-

tion-all the ingredients of Boyle's texture55-be regarded as in any
sense Aristotelian? One might reasonably suggest that this allegedly ves-
tigial scholasticism amounts to nothing more than what any materialist
would be likely to regard as a truism: that the macroscopic, observable
properties of a thing supervene on the microscopic properties. Of
course all the observable qualities of a thing flow from the underlying
texture of that thing (the arrangement of the thing's particles): that
just is the corpuscular hypothesis. In some authors, it seems plausible
to think that this is all that is meant. Malebranche, for instance, sug-
gests nothing more than this when he remarks that "there is nothing
wrong with the terms 'form' and 'essential difference.' Honey is
undoubtedly honey through its form, and in this lies its essential differ-
ence from salt. But this form or essential difference is only a matter of
the different configuration of its parts" (Search 1.16).56
In Boyle, however, there seems to be something more going on. In
replacing the scholastic term 'form' with his own 'texture', Boyle is
denying that forms are anything over and above the arrangement of
corpuscles that compose a body. But what he holds onto is the idea that
a unified body will have a single texture that explains the distinctive
and enduring qualities of that body. Thus, in introducing his term 'tex-
ture', he remarks:
And when many corpuscles do so convene together as to compose any dis-
tinct body, as a stone or a metal, then from their other accidents (or
modes), and from these last two mentioned [namely, posture and order],
there doth emerge a certain disposition or contrivance of parts in the
whole, which we may call the textureof it. (OFQ 30)
When Boyle speaks of a single texture of a distinct body, he suggests
something more than the trivial claim that for any aggregate of corpus-
cles we can speak of an aggregate of corpuscular facts, explaining the
properties of that aggregate. Now there are passages in Boyle, such as
the following, that might suggest just that sort of truistic aggregative
claim:

This convention of essential accidents, being taken (not any of them

apart, but all) together for the specifical difference that constitutes the
body and discriminates it from all other sorts of bodies, is by one name,
because considered as one collective thing, called its form. (OFQ52 n. 9)

Here Boyle seems eager to stress that the form is nothing more than
various characteristics of a body, taken all together. This may indeed be

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his considered view, and if so then he cannot be said to have preserved

the doctrine of substantial form in any significant sense. But there are
other passages where Boyle appeals to form or texture to account for
the unity of bodies. To the objection that bodies without a substantial
form can be only entia per accidens,he replies that this need not be the
case: that bodies can be unified when "matter, shape, situation, and
motion ordinantur per se et intrinsice to constitute one natural body"
(OFQ58). Boyle breaks into Latin here ("are ordered per se and intrin-
sically") as a way of ironically distancing himself from this kind ofjar-
gon. But the fact remains that he thinks the objection needs to be
answered and can be: he thinks his mechanistic approach can account
for the unity of bodies inasmuch as the disposition or texture of the
matter gives rise to the properties of the whole.57
If 'texture' here refers only to the corpuscular facts taken as a whole
(that is, to a conjunction of discrete facts), then he could hardly have
appealed to texture as that which unifies a coherent body. There would
be no way to distinguish between the texture possessed by a "distinct
body" and the texture possessed by, say, all the books and papers on top
of my desk. Now perhaps Boyle would welcome this last result, or per-
haps he was in fact torn over just how much weight to put on the tex-
ture of the whole as a unifying principle. But however one comes out
his particular case, what is clear again (as with Descartes) is that insofar
as modern authors reject Aristotelian substantial forms, they to that
extent face a difficulty in accounting for the unity of substances. If
Boyle has a response to scholastic arguments from unity, this is so only
insofar as he retains something significant from the scholastic scheme.
Even so, the most that can be said about Boyle is that the scholastic
conception of substantial form endured in part-with respect to its
concrete, causal side. With respect to the abstract, metaphysical side of
substantial form, Boyle rejected the doctrine, and did so in spectacular
fashion. For not only did he deny that the identity of a substance and
its parts is determined by the substantial form, but moreover he
rejected the entire scholastic conception of substantial identity and
change. So, contrary to the Aristotelian view that the parts of a sub-
stance maintain their identity only as part of that substance, Boyle
argues that when a body is generated, "no new substance is in genera-
tion produced, but only that which was pre-existent obtains a new mod-
ification or manner of existence" (OFQ 45). Likewise, when a body is
corrupted, it is "not that anything corporeal or substantial perishes in
this change, but only that the essential modification of the matter is

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destroyed" (OFQ 45-46). Although it is customary to speak of a body's

coming into or going out of existence-and so to this extent Boyle is
willing to retain the terms 'generation' and 'corruption'-in strict
metaphysical fact nothing ever does come into or go out of existence.
The real substances, the invisible corpuscles, endure over all change,
meaning that there is no real distinction between generation/corrup-
tion and alteration. This is just the sort of radical outcome that Buri-
dan, three centuries earlier, had described as a consequence of
reducing substantial form to a purely corpuscular account (see section
3).
Boyle repeatedly describes substantial forms as purely metaphysical
constructs, and hence not worthy of serious discussion.58 Yet, ironically,
his opposition to the theory rests in large part on his own views about
diachronic identity. Hence, although he has no use for the special sort
of substantial unity that the theory of substantial form was intended to
provide, this is not because he has turned his back on metaphysics, but
because he favors a rival metaphysics.

If Robert Boyle partly embraced the doctrine of substantial form, it

is clear thatJohn Locke goes much farther. Despite Locke's scorn for
"fruitless Enquiries" after "wholly unintelligible" substantial forms
(3.6.10), his own conception of a real essence or constitution contains
many central elements of the scholastic account. In the popular imag-
ination, Locke's theory of substance is held to rest on a bare substra-
tum, "a supposed, I know not what, to support those Ideas, we call
Accidents" (2.23.15). As is often observed, Locke does not positively
assert that this substratum has no properties (and in that sense is bare),
only that he himself is unwilling to speculate as to what those proper-
ties might be. Moreover, despite his pronounced caution with respect
to the notion of a substratum, Locke goes out quite far on a limb when
it comes to the doctrine of essence, taking for granted without argu-
ment that substances must have some unifying real essence, and then
going on to "presume" that this real essence should be understood
along the lines of Boyle's corpuscular account:
The particular parcel of Matter which makes the Ring I have on my Finger
is forwardly, by most Men, supposed to have a real Essence whereby it is
Gold;and from whence those Qualities flow, which I find in it, viz. its pecu-
liar Colour, Weight, Hardness, Fusibility, Fixedness, and change of
Colour upon a slight touch of Mercury, etc.This Essence, from which all
these Properties flow, when I enquire into it and search after it, I plainly

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perceiveI cannot discover:the farthestI can go is only to presumethat it

being nothing but Body, its real Essence or internal Constitution on
which these Qualities depend can be nothing but the Figure, Size, and
Connexion of its solid Parts.(2.31.6)
Locke's skeptical stance makes it hard for him to treat his corpuscular
account as anything more than a hypothesis.59 But he makes it clear
that the doctrine of real essences is not a mere hypothesis: "'tis past
doubt, there must be some real Constitution on which any Collection
of simple Ideas co-existing must depend" (3.3.15).60 This underlying
real essence gives rise to all the properties of the substance, with the
result that "it is as impossible, that two Things, partaking exactly of the
same real Essence,should have different Properties, as that two Figures
partaking in the same real Essenceof a Circle, should have different
Properties" (3.3.17).61 It is this, finally, that gives a substance its unity:
"the real Essence is that Constitution of the parts of Matter, on which
these Qualities, and their Union, depend" (3.6.6).
Reasonably enough, Locke treats his corpuscular interpretation of
essence as a departure from the scholastic conception of form. Thus,
he treats substantial form as a piece of obscurantism: "If any one will
say, that the real essence and internal constitution on which these
properties depend is not the figure, size, and arrangement or connex-
ion of its solid parts, but some thing else, called its particular form; I am
farther from having any idea of its real essence than I was before"
(2.31.6). But though the scholastics were not corpuscularians, we have
just seen that Locke's real essence plays very much the same causal role
that the substantial form played. Despite Locke's long association with
the doctrine of a bare substratum, his theory of substance is in fact
much closer to the scholastic view on which a substance is held
together by a single form that is causally responsible for its intrinsic
properties and nature.
If this is so, then why did Locke show such hostility toward the doc-
trine of substantial form? In part, as discussed already, this hostility was
based on a misconception of what the scholastic theory was; in part,
too, it was based on a real and substantive disagreement on the merits
of a strictly corpuscular conception of form. But there is something still
further here, a deep disagreement with the scholastics regarding the
relationship between substantial forms and our ideas of species mem-
bership. For the scholastics, our classification of individuals into spe-
cies tracks the essences of things. Though we may not have a direct or
comprehensive grasp of what those essences are (see section 4), we

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know enough to sort individuals into their true species. Like Boyle
before him,62 Locke utterly rejected this optimistic view, and took our
distinctions between species to be the product of haphazard and highly
fallible groupings on the basis of superficial resemblance: "the Species
of Things to us, are nothing but the ranking them under distinct
Names, according to the complex Ideas in us; and not according to pre-
cise, distinct, real Essences in them" (3.6.8). Even now, Locke says, we
cannot know real essences well enough to classify things properly. But
matters are even worse than that, because the general terms we use for
species and genera "have, for the most part, in all Languages, received
their Birth and Signification from ignorant and illiterate People, who
sorted and denominated Things by those sensible Qualities they found
in them" (3.6.25).
With this remark, Locke shows something of the profound differ-
ence in worldview that separates him from the medieval era. Medieval
authors took the creation story of Genesis (2:19-20) as having estab-
lished that it was Adam who gave a name to all living things (in Hebrew,
or so it was supposed). In this pre-fallen state, Adam was supposed to
have possessed cognitive abilities vastly superior to us fallen human
beings: hence, as Henry of Ghent put it, Adam would have gone about
his naming "as the ideal (optimus) metaphysician, knowing perfectly
the essences and quiddities of things, imposing various names on just
those species in keeping with the various essences and corresponding
to those very essences of the things."63We fallen descendants of Adam
have lost that talent for metaphysics, among much else, but the one leg-
acy we retain from Adam is a language scheme that truly cuts the world
at itsjoints. Even if we no longer speak Hebrew, we can readily compare
our own vulgar tongue to that Ur-language and see that our sortal
terms correspond generally to those of Hebrew.
Locke would evidently dismiss the bulk of that as nonsense. To the
extent that our language does map onto the real essences of things, he
thinks we have simply gotten lucky. And he offers a series of impressive
arguments against the optimism of the scholastics. He thinks it likely,
for instance, that the chain of being between species is smooth and
continuous, without the sorts of gaps and joints that facilitate classifi-
cation: "we shall find everywhere, that the several Species are linked
together, and differ but in almost insensible degrees" (3.6.12). Against
the standard scholastic position that we grasp something of the thing's
essence through its superficial properties, Locke argues that without
knowing the essence, we have no way of knowing which properties

66
 FORM, SUBSTANCE,AND MECHANISM

point toward the nature of the species and which are merely accidental
(3.6.19). Hence, our distinctions between species can be grounded
only on our haphazard conceptions of the nominal essences of things.
It is in this context that Locke utters his harshest words against substan-
tial forms, calling them "fruitless" and "unintelligible" (3.6.10), and
remarking,
But were there no other reason against it, yet the suppositionof Essences,
thatcannotbeknown;and the makingthem neverthelessto be that,which
distinguishes the Species of Things, is so whollyuseless,and unserviceable
to any part of our Knowledge,that that alone were sufficient,to make us
lay it by. (3.3.17)

Though Locke has other reasons to dislike substantial forms, this is the
point that he takes most seriously. And though this certainly is a serious
disagreement with the standard scholastic view, it is not a criticism of
the very notion of substantial form. Instead, what is at stake here is an
epistemological assumption that could easily have been attacked from
within the scholastic framework.
A complicating factor in this discussion is that Locke ties together
real and nominal essences. Although at times he suggests that each sub-
stance has its own, unique real essence, at other times he holds that a
thing's real essence is determined relative to the nominal essence
under which we conceive of the thing: "Essence, even in this [real]
sense, relates to a Sort, and supposes a Species. ... But that which
annexes them [real essences] still to the Species, is the nominal
Essence, of which they are the supposed foundation and cause" (3.6.6).
In support of this conclusion, Locke stresses that when he says that the
real essence gives rise to a thing's properties,he means that it gives rise
to those attributes that characterize every member of the species.
(Here, then, 'property' is being used in something like the traditional
sense of proprium.Thus, as quoted earlier, two figures with the real
essence circlewill have all the same "properties," though of course they
might differ in size, color, etc. [3.3.17].) Hence, the real essence of a
thing is tied to the species of the thing, and since we place a thing in its
species according to a nominal essence, the real essence is also depen-
dent on the nominal essence.64
This conception of real essence threatens to take us rather far away
from the scholastics. Instead of supposing that a given substance has a
unique essence from which all its intrinsic attributes flow (the view
described in section 2), Locke here seems to think that a given sub-
stance will have various real essences depending on how it is conceived,

67
 ROBERTPASNAU

each of which will be responsible for only those properties picked out
by a given nominal essence. Despite these scruples, however, it is clear
that Locke still accepts the scholastic notion of a thing's having a single
unifying nature that explains all of a thing's intrinsic characteristics.
Very often in this context, Locke speaks of a thing's "real constitution,"
which he contrasts with the "artificial constitution of genus and spe-
cies," which is the nominal essence. These are, Locke says, the two prin-
cipal senses of 'essence'. Describing the first, he writes:
First,Essencemaybe taken for the very being of any thing, wherebyit is,
what it is. And thus the real internal, but generally in Substances,
unknown Constitution of Things, whereon their discoverable Qualities
depend, may be called their Essence.This is the proper original significa-
tion of the Word. ... And in this sense it is still used, when we speak of the
Essenceof particularthings,withoutgivingthem any Name. (3.3.15)
The last sentence makes it clear that 'essence' in this sense belongs to
individuals, and belongs quite independently of our classifications.65
Such an essence gives rise, then, not only to certain properties con-
nected with a certain artificially imposed species, but quite generally to
its "discoverable qualities."66This essence, moreover, makes a thing be
"whatit is"-which is to say again not that it puts a thing into the species
or genus that we impose upon it, but that it makes the thing be what it
reallyis, whatever that is.
This last, unmistakably Aristotelian notion brings Locke back into
contact with the scholastic tradition. We have seen how Aquinas, for
instance, standardly distinguished between form as something held in
common by all members of a species, and an individual form that gives
rise even to the distinctive accidental properties of an individual. For
Aquinas, and for the scholastics in general, it is that individual form
that makes a thing be what it is, guaranteeing its unity at a time and
identity over time. All of this Locke accepts. What he does not accept,
as we have seen, is the characteristic scholastic optimism that we can
form abstract general accounts of species and genera that accurately
map the real differences between individual constitutions (read,
forms). But that, as I have said, is an epistemological issue that can be
severed from the doctrine of substantial form.
The lingering Aristotelianism I have identified in Locke falls on the
concrete side of substantial form. Locke's notion of real constitution
yields a physical hypothesis about the relationship between a thing and
its properties, and a causal account of why things tend to retain the
same properties over time. But this leaves open the metaphysical ques-

68
 FORM, SUBSTANCE,AND MECHANISM

tion of when we say that a thing remains genuinely the samething over
time, and when we say that it has become something different. Here
again, Locke's views were strongly influenced by Boyle, but in this case
the philosophical crudity of Boyle's account has been replaced by
something much more interesting and complex. Locke's most well-
known contribution to the metaphysics of identity comes in chapter 27
of book 2, where he offers a famous analysis of the diachronic identity
conditions for complex substances. Unlike simple substances (includ-
ing God, angels, and atoms), for which the identity conditions are
(according to Locke) straightforward and unproblematic, the analysis
for complex substances depends on the idea we have for that sub-
stance, "it being one thing to be the same Substance,another the same
Man, and a third the same Person"(?7).
Locke's famous account of personal identity-in terms of psycholog-
ical ties-is a wholly modem innovation. His account of the identity of
"substance" (by which he means a mass of matter or a body) is likewise
distinctive for being strictly mereological; Locke's rather surprising
view is that a mass of matter counts as the same mass (or same body)
just as long as it consists of the same parts: "whilst they exist united
together, the Mass, consisting of the same Atoms, must be the same
Mass, or the same Body, let the parts be never so differently jumbled"
(?3).67 Neither of these criteria makes any appeal to a thing's real con-
stitution, which might change or remain quite independently of what
happens at the psychological or mereological levels. Accordingly,
when we apply either of these criteria to a substance, we arrive at a very
different account from what an Aristotelian would offer. Masses of mat-
ter turn out to be changing constantly, as they gain or lose parts. Per-
sons can shuttle around between bodies, or go out of existence while
their bodies remain intact. It is plain, however, that neither of these cri-
teria gives a satisfactory account of the identity conditions for living
things, and it is here that Locke invokes something much like the doc-
trine of substantial form. In the case of plants, he argues, "that being
then one Plant, which has such an Organization of Parts in one coher-
ent Body, partaking of one Common Life, it continues to be the same
Plant, as long as it partakes of the same Life" (?4). Although Locke
does not say so at this point, it seems clear that this ongoing organiza-
tion is explained by the real constitution of the tree. When a sequence
of changing material parts is taken up into a continuously organized
aggregate, then "that continued Organization ... is fit to convey that
Common Life to all the Parts so united" (?4). Locke goes on to show

69
 ROBERTPASNAU

how much the same account can be given for animals (?5) and for
human beings (??6-8). As we have already seen, it is a thing's real con-
stitution that explains why it has the organization it has.
At this point, as with the scholastics, a physical conception of sub-
stance as sustained by a single concrete causal principle is joined to a
metaphysical conception of how that substance and its parts take their
identity conditions from being united with that principle. Thus, the
common life of a tree "has that Identity, which makes the same Plant,
and all the parts of it, parts of the same Plant" (?4). Now this is not to
say, by any means, that Locke has arrived at a full-blown scholastic con-
ception of substantial form. The doctrine of real constitution is playing
an analogous metaphysical role only at this onejuncture, and it is quite
uncertain how we are to understand the notion of sameplant, or same
man, when these are coupled with competing identity conditions for
same substance,or sameperson.So although Locke looks quite Aristote-
lian in his account of living things, he is undeniably very far from the
Aristotelians in treating that account as just one of several rival
accounts of identity.68
Still, focusing on the case of living things, we can see that a number
of interesting things have happened to the scholastic doctrine. First,
and most obviously, Locke severs the identity conditions for living
things from the identity conditions for soul. Even in the case of human
beings, he vigorously denies that "the Identity of Soul alone makes the
same Man" (?6). In divorcing soul from that which gives a thing life,
Locke accepts the Cartesian conception of soul as an immaterial sub-
stance (?21, 343.9-10), as against the Aristotelian conception of soul as
substantial form. Indeed, from this point forward in the history of phi-
losophy, the Aristotelian connection between soul and form would
become little more than an historical curiosity. To the extent that liv-
ing things can be said to have a form at all, that becomes some sort of
physical structure or organization.
This last observation points toward a second interesting transforma-
tion. Once form, understood as physical structure, enters into an
account of diachronic identity, it must be understood abstractly. Over
the course of an organism's life, it may well be that all the individual
particles will be replaced many times. What matters is that there be "the
same continued Life communicated to different Particles of Matter, as
they happen successively to be united to that organiz'd living Body"
(?8). What remains over time-the ongoing organization responsible
for continuing life-is not something real over and above the particles

70
 FORM, SUBSTANCE,AND MECHANISM

that come and go. (If it were, then this would be a fatal difficulty for
Locke's corpuscular account.) Instead, the endurance of a thing's real
constitution over time must be an abstraction, a universal, which is to
say (for Locke) that it is not a real thing in the world at all.69This marks
an important difference from the scholastic conception of substantial
form. For the scholastics, as we have seen, the substantial form was
understood as a real power, enduring through bodily change, over and
above the changing material components. If it is correct that this scho-
lastic picture marks something of a departure from the more abstract
conception of form to be found in Aristotle, then Locke can be seen as
moving at least somewhat in a more truly Aristotelian direction.
The abstractness of Locke's account is amplified in another way.
Whereas scholastic authors take the presence of a substantial form,
understood as a real power, to be precisely that which gives a thing its
identity over time, Locke's criterion focuses on something yet still
more abstract than an enduring, abstractly conceived, organization of
parts. What makes a plant or animal endure over time, for Locke, is its
being continuously alive, where to be alive gets spelled out in terms of
the characteristic functions of the living thing in question. In the case
of an oak tree, for instance, what matters is "such an Organization of
those parts, as is fit to receive and distribute nourishment, so as to con-
tinue and frame the Wood, Bark, and Leaves, etc. of an Oak, in which
consists the vegetative Life" (?4). Now Locke thinks that it is the thing's
enduring real constitution that accounts for this functional continuity.
But we could imagine function and constitution coming apart: the con-
stitution might change slightly, for instance, but not so much as to dis-
rupt the thing's ongoing life. Indeed, Locke stresses in various places
that the real constitutions of things can be changed, at least by God,
just as the inner mechanisms of a watch can be.70 Hence, although it is
plausible to assume that the identity over time of a living thing is deter-
mined by sameness in real constitution, this can have the status of noth-
ing more than an assumption; strictly, the identity conditions are
determined by sameness of function. Once again, Locke could be said
to be moving back in the direction of a more properly Aristotelian con-
ception of form. Substantial form plays a lingering role in his account,
at least in certain limited domains, but it has been transformed in var-
ious interesting and complex ways.

71
 ROBERTPASNAU

7. Conclusion

A clearer understanding of substantial forms and their history illumi-

nates some central philosophical questions. In particular, first, it points
toward a new (or, rather, old) way of thinking about substances, as
organized around a central unifying form. As for what sorts of things
would count as substances on this approach, or whether anythingwould
count as a substance, these are questions I have not addressed. As for
what such a form might itself be, the course of history suggests that
there are is wide range of options, from the strictly functional to the
strictly mechanistic. We should think notjust of a single theory, but of
a family of theories, some of which are no doubt preferable to others.
Second, the history of debate in this area shows how the concept of
form changed over time. Aristotle had introduced formal explanation
as a way of dealing with various puzzles that arose from ancient mate-
rialism. Modern authors returned to materialism at the expense of
Aristotelian metaphysics, and in particular abandoned Aristotle's for-
mal mode of explanation. But when one reads the middle chapters of
this story it begins to look as if formal explanation was already under-
going at least a shift in focus during the Middle Ages, and by the Renais-
sance had reverted to something much more like a material mode of
explanation. Hence some of the most significant changes in how form
has been understood seem to have occurred not at the hands of mod-
ern anti-Aristotelians, but with an earlier generation of scholastics. If
this is right, then those neglected middle chapters turn out to be piv-
otal for an understanding of form and substance in the modern era.
The demise of Aristotelian metaphysics might in part be laid at the
door of philosophers who were ostensibly Aristotelians.

University of Colorado at Boulder

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UncreatedBeing.Washington: Catholic University of America Press.

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Yolton, John. 1993. A LockeDictionary.Oxford: Blackwell.

Zavalloni, Roberto. 1951. Richardde Mediavilla et la Controversesur la Pluraliti
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Notes
I have benefited from discussing this material with a great many people in
various places. I owe particular thanks to audience members at the University
of California, San Diego, and to Peter Adamson, Jonathan Bennett, Jeremy
Buxbaum, Richard Cross, Chris Hughes, Dan Kaufman, John Marenbon,
Chris Shields, Martin Stone, Allen Wood, Paul Studtmann, and readers for the
PhilosophicalReview.
1 Most notorious of all is A. J. Ayer's dismissal of the debate over substance
as "spurious"and as based on "the primitive superstition that to every name a
single real entity must correspond" (1952, 40, 42). David Armstrong, though a
friend of substances, flatly remarks that "two different models compete for the
allegiance of philosophers. First there are Substance-attribute theories and
second, there are Bundle theories" (1989, 59).
2 See, respectively, Physics2.3 and Metaphysics7, esp. 1029a5-7.
3 See Averroes, Physics1.63 (4, 18vab); Aquinas, QDSC3c, ST la 77.6c.
4 See, for example, Aquinas, ST la 76.4c, CT 90.
5 See, for example, Franciscus Toletus, In Phys. 1.9.19 (4, 41ra); Eustachius
a Sancto Paulo, Summaphilosophiae,Physica 1.1.2.5; Aquinas, SCG 2.68.1450;
Coimbra, Physics1.9.10.1. For discussion, see Des Chene 1996 (65-66).
6 De rebusnaturalibus,421. Zabarella discusses and rejects various other cri-
teria.
7 "There is no reason why this is a substantial form and that an accidental
one, because propositions that are per se in the first mode do not have a rea-
son why. And just as this is per se in the first mode: white is a quality,and like-
wise a human beingis a substance,so too for this one: humanity,bywhicha human
being is a human being, is a substantialform" (QMet. 8.4.56). See also Ord.
4.11.3.44 (ed. Wadding 8, 648), and Cross 1998 (103-7).
8 See, for example, Stump and Kretzmann 1988 (285): "On Aquinas's view,
every thing has a substantial form. The substantial form of any thing is the set
of characteristics that place that thing in its species and that are thus essential
to it in Aquinas's sense of 'essential."' See also Cross 1998 (12): "A substantial
form, roughly speaking, is that property or set of properties in virtue of which
a material substance is a substance of such-and-such a kind," and Bennett 2001
(1:11): "The crucial explanatory fact about an organism [for Aristotle] is its
'form'. This is not a subset of the properties that the organism has, but rather
a set of those that are properto it, and towards which it strives or tends."
9 For example, Aquinas, ST la 29.2 ad 3, la 75.4c; Buridan, QMet.7.12.
Averroes was an exception to the standard medieval view: he thought the
essence could be identified with the form alone (see Met. 7.34 (8, 87ra)).
10Sufficientia 1.6 (f. 17rb). Avicenna distinguishes between accidents that
come from the matter and accidents that come from the form-following the
lead of Aristotle at, for example, Gen.An. 5.1.778a29-b2. This distinction does

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sometimes get repeated in Latin discussions, but would be downplayed-as we

will see-in favor of the view that all such accidents come from the substantial
form.
11Many other passages might be quoted, including I Sent. 17.1.2 obj. 2 & ad
2 (which expressly attributes the doctrine to Avicenna) and InDSS 15.229-31.
See also the discussion in Wippel 2000 (266-75). Often, the issue arises in
Aquinas's discussions of how the soul's powers (its accidents) flow (fluant)
from the soul's essence (a substantial form): see, in particular, ST la Q77.
Another context in which the issue frequently arises is in discussions of how
the motions of certain bodies follow from their forms (an upward motion fol-
lows from the form of fire, etc.): see, in particular, InPhys. 2.1.144 and ST
la2ae 17.9 ad 2. But some natural motions are not a product of the thing's sub-
stantial form: Aquinas gives the example of the ocean's tides (ST la 110.3 ad
1).
It is not entirely clear whether Aquinas takes such accidents to be the
product of the substantial form alone or of the whole essence, which would
include the substantial form and common matter. In some passages he speaks
only of the substantial form. Other passages refer explicitly to the essence or
else use phrases like 'substance' and 'principles of the substance', both of
which should be understood as referring to the essence as a whole. What
seems likely is that the form counts as the ultimate internal explanatory prin-
ciple, inasmuch as it accounts for why the proximate matter has the common
traits that it has.
There is a further question about whether certain accidents like eye color
might be the product not of the form, but of low-level material contingencies.
In the very early De ente,Aquinas follows Avicenna (see the previous note) in
remarking that "some accidents principally follow form, and some matter"
(6.60-62). But later works like the Summatheologiaeseem to take the view that
internal accidents flow from the substantial form alone. I suspect that this does
not mark any deep change in Aquinas's view. To the extent that matter at
some level does have an influence on the internal accidents of a substance, it
seems to do so at the start, when the substantial form and the matter come into
existence as a new substance. Thus, he remarks: "a form, considered in its own
right, is common to many particulars, but by its being received in matter it is
made the form, determinately, of this particular thing" (ST la 7.1c). To take a
concrete example, in explaining how two human beings can have qualitatively
different substantial forms (that is, rational souls), Aquinas remarks that this
can be a result of the matter: "A difference in form that comes solely from a
distinct disposition of the matter produces no distinction in species, but only a
numerical one. For distinct individuals have distinct forms, made distinct by
their matter" (ST la 85.7 ad 3). The view seems to be that when the rational
soul is infused into a certain embryo, the material dispositions of that embryo
have an influence on the soul itself (the substantial form), and so indirectly
have an influence on the internal accidents produced by the soul. Hence
Aquinas can say that all such accidents flow from the substantial form, and he
can also say that certain accidents are produced (indirectly) by the matter. I
discuss this issue at greater length in Pasnau 2002 (chap. 12), in connection

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with questions about individuation and personal identity.

12This list has to be drawn up with some care, because so many accidental
features of an individual are shaped at least in part by external causes.
Aquinas's own example, maleversusfemale, is ideal for his purposes, because it
picks out a nonessential attribute that has an entirely internal cause. Eye color,
I take it, is another plausible example. But how many other such examples are
there? And, in general, is there a clear external and internal division between
accidents? Consider another of Aquinas's examples, skin color. It is obviously
not a proprium, but can it be considered an internal accident that stems from
the distinctive form of an individual? There is some temptation to say so, inas-
much as skin color obviously has an innate component. On the other hand,
skin color is readily changeable, within certain limits. Does it make any sense,
then, to say that this particularskin coloris a product of my substantial form?
Probably not. And the same issues arise for many other accidental properties,
such as weight, height, musical ability, and much else.
Aquinas suggests a solution, however. He claims that "every natural body
has some determinate substantial form. Therefore, since the accidents follow
from the substantial form, it is necessary that determinate accidents follow
from a determinate form" (ST la 7.3c). In saying that the accidents are deter-
minate, Aquinas means not that they are precisely set, but that they are bound
within certain limits. We can be sure that this is what he means, because the
previous article (7.2c) had just told us that "matter, inasmuch a, it is under
one substantial form, remains in potentiality to many accidental forms," giving
it a kind of potential infinity. His example is of a piece of wood that has a
potentially infinite number of shapes. The only way to make these passages
consistent is to understand the role of the substantial form as imposing a cer-
tain range of options on an individual. Taken alone, the substantial form rules
out certain possibilities, like being ten feet tall or three feet tall, but within a
certain range leaves open a virtually infinite number of possibilities. (And see
InDA 2.8.159-71, where this line of thought is explicitly developed.) So we
might say, by way of elucidating the canonical Aristotelian example, that if
albus means pale, then it is an external accident for a human being, inasmuch
as it depends on where you live, what you wear, and what you do. If, on the
other hand, albus means white,then (on the modern English idiom, and put-
ting aside various cultural complications) it is an internal accident, inasmuch
as it refers to one's innate pigmentation.
13Although this passage seems to commit Aquinas to individual essences,
there are reasons to doubt whether, strictly speaking, he thinks that individu-
als within a species can have qualitatively distinct essences.(For instance, the
essence of a thing is given by its definition; but individuals cannot be defined.)
Still, there is no room for doubt that Aquinas thinks of individuals within a spe-
cies as having qualitatively distinct substantial forms. This would make it natu-
ral for him to speak, on occasion, as if they had different essences.
14This book-length treatise has recently been translated by Kronen and
Reedy, with useful and extensive notes.
15For discussion of the particular argument, see Des Chene 1996 (73-75).
In general, early modern scholars seem better informed today than medieval-

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ists on the topic of substantial form, probably because the doctrine is stressed
more in Renaissance texts. The fullest recent discussion is Des Chene 1996,
which remarks that "essence, if it is identified with substantial form, is not a
mere list of properties the loss of any one of which must result in the destruc-
tion of the individual" (71). See also McCann 1987 (55-57), Rozemond 1998
(104), and Ariew and Gabbey 1998: "The substantial form is a determinative
active principle informing and conferring essence on matter, defining the
resulting substance, and locating it in its class or species. .... Furthermore, the
substantial form yields the sensible and insensible qualities (qualitates) pos-
sessed by the substance in question and is the immediate cause of the phenom-
ena that are characteristic of it" (430).
161 owe these references to Wippel 1981 (176-84).
17If we think of souls as the paradigmatic substantial forms, then it is
espe-
cially natural to think of them as being or having causal powers. See, for
instance, Shields 2003 (supplement 2), which stresses the role of soul as the
source of perception and thought, and as an efficient cause of motion (for
example, De an. 2.4.415b8-28). Cooper 1988 makes an analogous point from
the perspective of Aristotle's embryology:
But [the formof an animal]is directlyresponsiblenot only for its havingall the
tissues,organsand limbsessentialto a humanbeing, but also for manyindivid-
ual featuresof the waythese are found constitutedand arrangedin thatpartic-
ular animal. Roughly,these will be all those featuresthat, as Aristotlethinks,
cannot successfullybe explained as due either to environmentalinfluences or
to incidentalpropertiesof the matterthat goes to constituteand sustainthem.
(37)
This is strikingly akin to the later scholastic conception of substantial form, as
described in the previous section.
18According to Irwin 1980, "A natural substance's form is its characteristic
function rather than its structure or composition, which are features of its mat-
ter" (38). But compare D. C. Williams, who remarks of the Aristotelian link
between form and function that "no ties in the system are flimsier than this"
(1958,309).
19Aquinas usually makes this point in the context of the soul. But he always
makes it clear that the same holds for all and only substantial forms. See ST la
76.8, discussed below, and also QDA 10, QDSC4c, InDA 1.14, ST la 8.2 ad 3, I
Sent.8.5.3.
20See, for example, Meteorology 4.12.389b31-390a19; De an. 2.1.412b20-22;
Shields 1999, chap. 5.
21Aquinas regularly denies that artifacts count as substances: see, for exam-
ple, SCG 1.18.141, 4.35.3731; ST la 76.8, la2ae 17.4c; InDA 2.1.157-58; QQ
11.6 ad 3. But he does at least once, at ST 3a 75.6 ad 1, allow that substantial
forms can be made artificially. In that passage he first gives the examples of
frogs and snakes (referring to Exodus 7-8, where Moses performs his miracles
and then-crucially for Aquinas's point-they are matched by the Pharaoh's
magicians). Then he goes on to give the more mundane example of a loaf of
bread, made by baking flour and water. In all these cases, "the art produces
such a form not by its own power, but by the power of natural principles." If he
means here that the whole loaf is a single substance, then I have a hard time

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seeing how to square this example with his broader theory. (He would have to
think that slices of bread lose their identity.) In general, though, notwithstand-
ing his usual assertions, it should not be surprising that artificial substances are
possible. His theory of substantial form explains precisely under what circum-
stances an artifact would have the requisite unity. For an illuminating discus-
sion of Aquinas on artifacts, see Rota 2004.
22See ST la 13.5c, where signum is contrasted with causa: "hoc quidem
signum est, illud vero causa." 'Signum' in this sense occurs dozens of times in
the first part of the Summa theologiaealone. See, for example, 75.6c, 76.8c,
84.7c. For a more general discussion of the plausibility of Aquinas's metaphys-
ics, see Pasnau 2002 (sects. 3.2-3).
23Still, there is ample room for interpretation here, especially since Aristo-
tle goes on to remark that, for the parts of substances, "they all are what they
are in virtue of a certain power of action or passion" (390a18)--suggesting
that perhaps the form is not the function but the power for performing the
function, and so perhaps drifting back toward the concrete side of form.
24Recent literature in this area has tended to concentrate on the question
of whether forms are individuals or universals. Although that issue is certainly
connected to the issue I am raising, they are separate questions and deserve to
be treated as such. Even if forms are universal, there is still a question of
whether they should be conceived of as causally active. And even if forms are
particulars, one might still think of them as metaphysical abstractions rather
than actual causal powers.
25Hence, Balme could not be more wrong when he writes: "The extraordi-
nary later misinterpretations of Aristotle, the magical entelechies and real spe-
cific forms, must be largely due to these imported concepts-Species,
Essentia, Substantia-which presided like three witches over his rebirth in the
Middle Ages, but should be banished to haunt the neoplatonism from which
they came" (1987, 306). The actual differences between Aristotle and his scho-
lastic followers lie in precisely the opposite direction. Far from treating forms
and essences as magical platonic entities, the scholastics tended to conceive of
these entities in highly naturalistic and empirical ways, along just the sorts of
lines that Balme himself extols.
26See the discussion in Ariew 1992 (63-69) of the restrictions placed on
Jesuit philosophers and theologians. As Ariew remarks, "the Jesuits defined
danger to the faith as any novelty in either theology or in philosophy, espe-
cially as it concerned the axioms and common opinions of scholasticism" (65).
27Thus, Descartes tells Mersenne, in correspondence from 1640 (which
postdates his most famous philosophical works), that he has not read scholas-
tic philosophy in twenty years. He then adds, "I would also like to know
whether there is someone who has written a summary of all of scholastic phi-
losophy and who has a following, for this would spare me the time to read their
big books" (AT 3:185; CSMK, 154).
28See Henry of Ghent, Quod. 4.13 (f. 104vl); Ockham, Quod. 3.6 (OTh
9.227), and Ord.8.3 (OTh 3.206); Aquinas, De ente 5.76-81, ST la 77.1 ad 7,
QDSC 11 ad 3, InPA 2.13.119-21 [sect. 533], and InDA 1.1.254-55; Scotus,
QMet.7.3.16, Lectura1.22.2-4 (Balik 17.301-2), Ord.1.22.6 (Balic 5.344).

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29See, for example, Mackie 1976 (86-88), Yolton 1993 (68).

30See also Scotus, QMet.7.8-10, 59; Zabarella, De rebusnat. 385c-f; Suirez,
DM 15.1.16, 15.2.10, 15.4.4.
31See AT 3:232 (CSMK, 156); cf. AT 3:185 (CSMK, 154).
32Gilson 1951 (162-63, 170-71) stresses that Descartes's theory has no
room for real entities that are not substances; Des Chene 1996 (78-79) points
to the Aristotelian distinction between primary and secondary substances;
Rozemond 1998 (130-33) argues that substantial forms are separable from
matter (according to the scholastics), and hence substances (according to
Descartes).
33See also OFQ150, where Boyle describes substantial forms as immaterial
substances. Tellingly, Dutch translations of Spinoza render 'formasubstantialis'
as 'zelfstandigevorm'--that is, a subsistentform (ed. Curley, 678). A better choice
would have been 'wezenlijkvorm'.Another, fairly bizarre instance is Hobbes,
who identifies substantial forms with "certain essences separated from bodies"
(Leviathan 46.15) and then remarks that "this doctrine of separatedessences,
built on the vain philosophy of Aristotle, would frighten them [men] from
obeying the laws of their country with empty names, as men fright birds from
the corn with an empty doublet, a hat, and a crooked stick" (46.18). Eventu-
ally, it becomes clear that Hobbes is conflating various scholastic views: that
the rational soul is subsistent; that the accidents in the Eucharist can miracu-
lously endure without their substance; and that virtues can be infused by God.
Frightening as these doctrines may or may not be, only the first has anything to
do with substantial forms, and it is in fact an exceptional case that goes against
the general rule regarding substantial form.
34See, for example, the Fourth Set of Replies (CSM 2:157; AT 7:222), and
also a letter to Regius, Dec. 1641 (CSMK, 200; AT 3:460).
35See also Scotus, QPraed15.10, who stresses that 'substance' is equivocal,
and that form and matter count as substances only inasmuch as they are prin-
ciples of substance.
36The diversity of views is well brought out in Mercer 1993 and Fitzpatrick
2003 (308-16). For a discussion of seventeenth-century criticisms of substan-
tial form before and after Descartes, see Ariew 1999 (77-96). The best-known
attempt to defend some sort of middle ground is of course Leibniz, especially
in his correspondence with Arnauld (see Sleigh 1990). I discuss Leibniz briefly
near the start of section 6.
37All translations from Descartes are based on Cottingham et al., with some
alterations. On there being just one kind of matter, see, for example, Meteorol-
ogy,Discourse I (AT 6:239; CSM 1:187 n.); The World,chap. 4; Principles3.46.
On the sorts of differences that affect it, see, for example, The World,chap. 5
(AT 11:25-26; CSM 1:89); Principles4.187, 4.198. For a useful summary of how
Descartes's physics differs from scholasticism as he conceives of it, see Garber
2001 (224-31).
38These two lines of attack-from the poverty of scholastic explanations
and from the richness of his own--come together in a remark added to the
French edition of Principles4.201, that prime matter, substantial form, etc. "are
harder to understand than all the things they are supposed explain." For fur-

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ther disparaging remarks, see CSMK, 107, 122, 188, 221; AT 8B:26; AT 1:430.
The passages in the main text are offered as suggestions regarding how Regius
should reply to criticism at the University of Utrecht. Although I see no reason
to doubt that Descartes held the views he sets out in this letter, his words
should be placed within the proper rhetorical context, as Chappell 1994
stresses.
39See AT 8B:351 (CSM 1:299); AT 3:460 (CSMK, 200).
40See, for example, his exchange with Amauld on whether a sheep can see
a wolf and run awaywithout a rational soul (AT 7:205, 230; CSM 2:144, 161), as
well as AT 11:202 (CSM 1:108), AT 1:414-15 (CSMK, 62), and AT 2:40-41
(CSMK,99-100). Morris 2000 neatly captures the inference: "Thus his aim was
to explain sentiencewithout recourse to a sensitive soul. Success in doing so, if
combined with Ockham's razor, would imply that animals did not have 'any
vegetative or sensitive soul"' (402-3).
41Earlier in this same letter, Descartes remarks that "the human soul alone
is recognized as a substantial form, whereas other forms consist in the config-
uration and motion of their parts" (AT 3:503; CSMK, 207). Elsewhere, Des-
cartes characterizes the mind as the form of the body, or as informing the
body: see AT 10:411 (CSM 1:0); AT 7:356 (CSM 2:246); AT 4:168 (CSMK, 243-
44); AT 4:346 (CSMK, 279); AT 4:373 (CSMK,284). The original Latin text of
Principles4.189 speaks of the soul's " informing the entire body," but the sub-
sequent French translation has the soul's being "united to the entire body."
Voss 1994 contains a very useful cataloguing of these and other crucial texts
relating to the mind-body problem. But given that these passages extend from
the Regulaeof 1628 to correspondence in 1646, there seems little reason to
accept Voss's argument that this reflects a "brief period" of scholastic thinking
in Descartes's career (277). This makes too little of this language, by confining
it to a two-yearperiod, and also-as we will see-makes too much of it, by sup-
posing that within that period Descartes was seriously under the influence of
this scholastic scheme.
42See Hoffman 1986, Voss 1994, Des Chene 1996, Rodis-Lewis 1998, Roze-
mond 1998.
43See Principles4.189, Med. 6 (AT 7:86; CSM 2:59), Fifth Replies (AT 7:388-
89; CSM 2:266). In making these claims, Descartes is doing his best to adhere
to a long tradition that goes back through Augustine (for example, De trinitate
6.6.8) to Plotinus (Ennead 4.2.1).
44Rodis-Lewis 1998 seems to give the doctrine this kind of metaphysical
weight (206). Compare M. Wilson (Garber and Wilson 1998, 836), for whom
the claim means nothing more than that the mind moves (is moved by) the
pineal gland, which in turn moves (is moved by) the whole body. As will
emerge, I am more sympathetic to Wilson's reading.
45Descartes reiterates this claim in the same letter (AT 4:167), and again in
a subsequent letter to Mesland (AT 4:346; CSMK, 278-79). Hoffman 1986
gives a prominent place to these passages. In reply, Chappell 1994 argues that
this claim "contains nothing specifically Aristotelian" (417), a conclusion I
hope we are now in a position to reject.
46See Rozemond 1998: "Descartes simply never proposes that the mind is

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the form of the body as an accountof theirunion" (152).

47Rozemond 1998, after making some of these same points, concludes that
"Descartes does not propose to offer a hylomorphic explanation of the unity
of the human being" (163). As for apparent indications otherwise: "He did
adapt various expressions from the Aristotelian scholastic tradition, which he
only uses when under pressure from opponents" (162). This last remark is
overstated, inasmuch as Descartes's boldest pronouncements in this area were
to Mesland, an enthusiastic supporter, and were not made under pressure. But
what seems correct is that Descartes did not suppose hylomorphism could
explain the unity of mind and body. Rozemond goes on, quite correctly, to
stress the influence of Church doctrine, as promulgated in the Lateran Coun-
cil of 1513, which dictated that the rational soul is the form of the human
body. For the medieval background to that doctrine, see Pasnau 1997.
48An important further element in this discussion, which here I can only
note, is the connection between Descartes and those scholastics who postu-
lated multiple substantial forms within a human being. For key medieval fig-
ures like Ghent, Scotus, and Ockham, a human being has, in an additional to
the rational soul, at least one other substantial form that is the form of the
body. Although this pluralist account was the minority opinion by the time of
the sixteenth century, it is still worth considering whether Descartes's view
might be consistent with it. And although the argument of the main text shows
that the Cartesian mind is not the form of the body in the standard scholastic
sense, the mind might still count as a substantial form in something like the
way it did for these medieval pluralists. This conclusion has been advanced by
a number of scholars, including G. Wilson (1982), Hoffman (1986, 363-64),
Des Chene (1996, 65), and Rozemond (1998, 145). Though I cannot discuss
pluralism at length here, I hope it is clear at any rate just how little content
there is left in Descartes's claim that the mind is the form of the body.
49See, for example, Locke, Essay2.23.29-30. Ayers remarks that "the key to
understanding Locke's general theory of substance is to realize that it is noth-
ing other than a restatement and elaboration of the sceptical position adopted
by Gassendi" (1991, vol. 2, 31).
50See, in particular, Emerton, who remarks that "the opponents of scholas-
ticism ... did not usually reject the concept of form as such, and in fact the
denials of the form, however vehemently stated, were more apparent than
real" (1984, 60; see also 72).
51See also Discourse12, where Leibniz remarks that, although substantial
form is needed, "it makes no change in the phenomena, any more than do the
souls of animals, if they have any."To Arnauld he writes: "All the phenomena
of bodies can be explained mechanically, that is, by the corpuscular philoso-
phy, following certain principles of mechanics posited without troubling one-
self over whether there are souls or not" (ed. Ariew and Garber, 80).
Garber aptly remarks that Leibniz introduces substantial form "not to dis-
rupt a perfect mechanism or to explain what cannot be explained mechani-
cally, ... but to ground mechanism" (1998, 784).
52 "For I think it is a mistake to imagine (as we are wont to do) that what is
called the natureof this or that body is wholly comprised in its own matterand

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its (I say not substantial,but) essentialform, as if from that or these only all its
operations must flow" (ed. Stewart, 190). Boyle goes on to make the interest-
ing claim that such operations flow from matter and form only given a certain
environment. In his Lexicon Philosophicum (originally published in 1692),
Stephanus Chauvin writes that "although no substantialforms are posited, still
essentialforms are posited," which he goes on to define as "that through which
each body is established in a certain species, is distinguished from all others,
and operates in a way appropriate to its nature" (1713, 261a). The history of
this subject might well have been dramatically different if, from the start, the
scholastics had used the far more apt phrase forma essentialis.(This is just one
more respect in which the standard translation of ousia as substantia (and sub-
stance) has led to endless confusion.)
53For specific details regarding Boyle's chemical conclusions in this regard,
see Emerton 1984 (143-46). Like Descartes, Boyle explicitly indicates that his
attack on substantial form does not apply to "the reasonable [that is, rational]
soul that is said to inform the human body" (OFQ 15).
54See OFQ passim-for example, 30, quoted later in the main text. The
term goes back at least to Francis Bacon: "the true textures and configurations
of bodies on which all the occult and, as they are called, specific properties
and virtues in things depend, and from which, too, the rule of every powerful
alteration and transformation is derived" (New Organon2.7).
55See, for example, OFQ 18-20, 30, 71. For further discussion, see Alex-
ander 1985 (60-88). Chauvin's entry for 'form' remarks that "the form of a
natural body, according to what more recent philosophers hold, is nothing
other than a mode of the matter in which it inheres," and he goes on to
describe five modes: shape, rest, motion, size, position (1713, 260b).
56Perhaps another example is Arnauld and Nicole, in the Port-RoyalLogic
"the form is whatever makes something to be what it is and distinguishes it
from other things, whether it is a being really distinct from the matter, as the
scholastics think, or merely the arrangement of its parts" (187-88).
57For a detailed illustration of this strategy, see the discussion in OFQ65-
66. Boyle there considers the argument that substantial forms are required to
explain "the various changes ... the differing effects ... the preservation and
restitution of the state requisite to each particular body, as also the keeping of
its several parts united into one totum."He replies that "many and great alter-
ations may happen to bodies, which seem manifestly to proceed from [1] their
peculiar texture and [2] the action of outward agents upon them." Then, in
exploring the first case, he writes that "various operations of a body may be
derived from the peculiar texture of the whole and the mechanical affections
of the particular corpuscles or other parts that compose it," and he goes on to
describe the case of vitriol produced in his laboratory, which has "most, if not
all" of the properties possessed by natural vitriol. Boyle takes this to be devas-
tating for the doctrine of substantial forms, because he assumes that his artifi-
cial vitriol will not be said to have a substantial form. Its properties, he says,
"plainly emerge" from the underlying texture of the matter. But, he then rea-
sons, surely natural vitriol should likewise be said to emerge from the underly-
ing texture. (For further discussion of this particular example, in the context

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of a general argument against putting weight on the natural-artificial distinc-

tion, see OFQ 74-79.) The proponent of substantial forms should probably
reply that artificial vitriol has a substantial form. This is the tack that Aquinas
suggests in note 22 above.
58See OFQ57-58, and "History of Firmness" ( Works1:261a): "to engage very
far in such a metaphysical and nice speculation were unfit for me."
59Even so, Locke sometimes endorses such an account without any qualifi-
cations or hesitations, for example: "... Bulk, Texture, and Figure of the
minute parts of Bodies, on which their real Constitutions and Differences
depend..." ( 2.23.8). Unless otherwise indicated, references to Locke are to
the Essay (ed. Nidditch), usually to book, chapter, and section, and occasion-
ally to page and line number. Quotations sometimes modernize Locke's punc-
tuation and ignore italics.
60See also the first letter to Stillingfleet: "I know nobody that ever denied
the certainty of such real essences or internal constitutions, in things that do
exist" (Works4:82); "I easily grant there is reality in them; and it was from that
reality that I called them real essences" (ibid., 83).
613.6.7 is virtually identical. See also 3.3.18 and 3.6.2, both of which hold
that all the properties of a substance "depend" on its real essence.
62See, for example, OFQ72: "it was very much by a kind of tacit agreement
that men had distinguished the species of bodies. ... those distinctions were
more arbitrary than we are wont to be aware of."
63Lectura 206. See Dahan 1995 for further references and discussion.
Aquinas treats Adam's naming of the animals as an argument for Adam's vir-
tual omniscience. He argues: "Adam gave names to the animals, as is said at
Genesis 2. But names ought to fit with the natures of things. Therefore Adam
knew the natures of all animals, and by parity of reason, had knowledge of all
other things" (ST la 94.3sc). (The conclusion gets qualified in the main reply,
so that it extends only to omniscience regarding what it is appropriate for a
human being to know about-thereby excluding things such as the thoughts
of other human beings, future contingents, and trivial facts such as how many
pebbles lie in a certain stream.)
64Many recent commentators have stressed this point: see, in particular,
McCann 1987 (64-67), Ayers 1991 (2:73-74), Guyer 1994. Often enough,
however, Locke ignores this purported connection between real and nominal
essence: see, for example, 3.6.19-20, where he is quite clearly thinking of a
substance as having a unique real essence, giving rise to just those properties
that are truly essential to a thing. We can never discover this real essence, he
argues there, because "it is impossible to know all those Properties, that flow
from it, and are so annexed to it, that any one of them being away,we may cer-
tainly conclude, that that Essence is not there, and so the Thing is not of that
Species" (sect. 19). If the real essence were fixed by the nominal essence, in
the way Locke describes in 3.3.6, then we would be able simply to stipulate
what the essential properties are.
65See also the first letter to Stillingfleet: "I grant it true, what your lordship
says in the next words, 'and let the nominal essences differ never so much, the
real, common essence or nature of the several kinds, is not at all altered by

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them;' i.e. that our thoughts or ideas cannot alter the real constitutions that
are in things that exist; there is nothing more certain" ( Works4:90); "It is true,
the real constitutions or essences of particular things existing, do not depend
on the ideas of men, but on the will of the Creator; but their being ranked into
sorts, under such and such names, does depend, and wholly depend, upon the
ideas of men" (91).
At the end of 3.3.15, Locke identifies a thing's real constitution, so con-
ceived, with the thing's real essence. So that looks to be another case (see pre-
vious note) where Locke is ignoring the alleged tie between real and nominal
essence.
66See Ayers 1991 (2:73). "For if we consider the 'real essence' to be the
underlying structure of the particular, without reference to a name, it has to
be considered as a whole; and then it will appear that all the qualities and nat-
ural attributes of the thing, whether classed by us as 'difference', 'properties',
or 'accidents', flow equally from the 'real essence'."
67Locke's account of bodily sameness, and his conviction that questions of
identity somehow depend on the idea one has of a thing, closely follow Hob-
bes's treatment of these matters at De corpore2.11.7. Boyle had likewise stressed
how identity judgments depend on the ideas we employ (ed. Stewart, 193-94).
68It would be desirable at this point to have a clear understanding of how
Locke understands these rival accounts-in particular, whether they require a
conception of relative identity, or whether for a given individual there is one
privileged criterion of absolute identity, with the other criteria furnishing
identity only in some looser sense. The difficulties with relative identity are
well known and have been much discussed, both in general and as a reading of
Locke. See, in general, Perry 1970 and Wiggins 1980 (chap. 1). For the case of
Locke, see Alston and Bennett 1988, Chappell 1989, Lowe 1995 (chap. 5).
Here, unfortunately, I must set aside this question entirely.
69"[U]niversality belongs not to things themselves, which are all of them
particular in their Existence" (3.3.11).
70See the first letter to Stillingfleet: "Of what, I beseech your lordship, are
the internal constitutions unchangeable? Not of any thing that exists, but of
God alone; for they may be changed all as easily by that hand that made them,
as the internal frame of a watch" (Works4:91).

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