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{{Short description|Property of a statement that can be logically contradicted}}{{Multiple issues|{{Citation style|date=September 2024}}
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[[File:Black Swans.jpg|thumb|upright=1.3|alt=Pair of black swans swimming|Here are two [[black swan]]s, but even with no black swans to possibly falsify it, "All swans are white" would still be shown falsifiable by "Here is a black swan"—a black swan would still be a state of affairs, only an imaginary one.<ref name=Popperonstateofaffairs group="upper-alpha"/>]]
'''Falsifiability''' (or '''refutability''') is a [[deductive]] standard of evaluation of scientific theories and hypotheses, introduced by the [[Philosophy of science|philosopher of science]] [[Karl Popper]] in his book ''[[The Logic of Scientific Discovery]]'' (1934).{{refn|group=upper-alpha| name=faithfultranslationofLoSD}} A [[Scientific theory|theory]] or [[hypothesis]] is
Popper emphasized the asymmetry created by the relation of a universal law with basic observation statements{{refn|group=upper-alpha|name="basicstatementsbreakthesymmetry"|The falsifiability criterion is formulated in terms of basic statements or observation statements without requiring that we know which ones of these observation statements correspond to actual facts. These basic statements break the symmetry, while being purely logical concepts.}} and contrasted falsifiability to the intuitively similar concept of [[Verifiability (science)|verifiability]] that was then current in [[logical positivism]]. He argued that the only way to verify a claim such as "All swans are white" would be if one could theoretically observe all swans,{{refn|group=upper-alpha|name="blackswanimpossible"}} which is not possible. On the other hand, the falsifiability requirement for an anomalous instance, such as the observation of a single black swan, is theoretically reasonable and sufficient to logically falsify the claim.
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The problem of induction is often called Hume's problem. [[David Hume]] studied how human beings obtain new knowledge that goes beyond known laws and observations, including how we can discover new laws. He understood that deductive logic could not explain this learning process and argued in favour of a mental or psychological process of learning that would not require deductive logic. He even argued that this learning process cannot be justified by any general rules, deductive or not.{{sfn|Thornton|2007}} Popper accepted Hume's argument and therefore viewed progress in science as the result of quasi-induction, which does the same as induction, but has no inference rules to justify it.{{sfn|Popper|1959|loc=Sec. 85}}{{sfn|Watkins|1984|loc=Sec. 7.2}} [[Philip Johnson-Laird|Philip N. Johnson-Laird]], professor of psychology, also accepted Hume's conclusion that induction has no justification. For him induction does not require justification and therefore can exist in the same manner as Popper's quasi-induction does.{{sfn|Johnson-Laird|2006|loc=Chap. 13}}
When Johnson-Laird says that no justification is needed, he does not refer to a general inductive method of justification that, to avoid a circular reasoning, would not itself require any justification<!-- as is hoped for in sophisticated falsificationism (see {{section link|Falsifiability|Sophisticated falsificationism: a natural search for a method of justification}})-->. On the contrary, in agreement with Hume, he means that there is no general method of justification for induction and that's ok, because the induction steps do not require justification.{{sfn|Johnson-Laird|2006|loc=Chap. 13}} Instead, these steps use [[Inductive reasoning|patterns of induction]], which are not expected to have a general justification: they may or may not be applicable depending on the background knowledge. Johnson-Laird wrote: "[P]hilosophers have worried about which properties of objects warrant inductive inferences. The answer rests on knowledge: we don't infer that all the passengers on a plane are male because the first ten off the plane are men. We know that this observation doesn't rule out the possibility of a woman passenger."{{sfn|Johnson-Laird|2006|loc=Chap. 13}} The reasoning pattern that was not applied here is [[Inductive reasoning#Enumerative induction|enumerative induction]].
Popper was interested in the overall learning process in science, to quasi-induction, which he also called the "path of science".{{sfn|Popper|1959|loc=Sec. 85}} However, Popper did not show much interest in these reasoning patterns, which he globally referred to as psychologism.{{sfn|Popper|1959|loc=Sec 2}} He did not deny the possibility of some kind of psychological explanation for the learning process, especially when psychology is seen as an extension of biology, but he felt that these biological explanations were not within the scope of epistemology.<ref group="upper-alpha" name="Popperpsychologyshouldbebiology"/><ref group="upper-alpha" name="Popperepistemologywithoutbiology"/> Popper proposed an evolutionary mechanism to explain the success of science,{{sfn|Popper|1972|loc=App. 1.III}} which is much in line with Johnson-Laird's view that "induction is just something that animals, including human beings, do to make life possible",{{sfn|Johnson-Laird|2006|loc=Chap. 13}} but Popper did not consider it a part of his epistemology.{{sfn|Popper|1972|loc=App. 1.II}} He wrote that his interest was mainly in the ''logic'' of science and that epistemology should be concerned with logical aspects only.<ref group="upper-alpha" name="Popperagainstpsichologism"/> Instead of asking why science succeeds he considered the pragmatic problem of induction.{{sfn|Popper|1972|loc=Sec. 1.9}} This problem is not how to justify a theory or what is the global mechanism for the success of science but only what methodology do we use to pick one theory among theories that are already conjectured. His methodological answer to the latter question is that we pick the theory that is the most tested with the available technology: "the one, which in the light of our ''critical discussion'', appears to be the best so far".{{sfn|Popper|1972|loc=Sec. 1.9}} By his own account, because only a negative approach was supported by logic, Popper adopted a negative methodology.<ref name="Popperabouthisnegativemethodology" group="upper-alpha">{{harvnb|Popper|1972|loc=Sec. 1.8}}: "The fundamental difference between my approach and the approach for which I long ago introduced the label 'inductivist' is that I lay stress on negative arguments, such as negative instances or counter-examples, refutations, and attempted refutations—in short, criticism".</ref> The purpose of his methodology is to prevent "the policy of immunizing our theories against refutation". It also supports some "dogmatic attitude" in defending theories against criticism, because this allows the process to be more complete.{{sfn|Popper|1972|p=30}} This negative view of science was much criticized and not only by Johnson-Laird.
In practice, some steps based on observations can be justified under assumptions, which can be very natural. For example, Bayesian inductive logic{{sfn|Gelman|Shalizi|2013|}} is justified by theorems that make explicit assumptions. These theorems are obtained with deductive logic, not inductive logic. They are sometimes presented as steps of induction, because they refer to laws of probability, even though they do not go beyond deductive logic. This is yet a third notion of induction, which
==The elusive distinction between the logic of science and its applied methodology==
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So observations have two purposes in Popper's view. On the methodological side, observations can be used to show that a law is false, which Popper calls falsification. On the logical side, observations, which are purely logical constructions, do not show a law to be false, but contradict a law to show its falsifiability. Unlike falsifications and ''free from the problems of falsification'', these contradictions establish the value of the law, which may eventually be corroborated.
Popper wrote that an entire literature exists because this distinction between the logical aspect and the methodological aspect was not observed.<ref name="somecontradiction" group="upper-alpha"/> This is still seen in a more recent literature. For example, in their 2019 article ''Evidence based medicine as science'', Vere and Gibson wrote "[falsifiability has] been considered problematic because theories are not simply tested through falsification but in conjunction with auxiliary assumptions and background knowledge."<ref>{{Cite journal |last1=Vere |first1=Joseph |last2=Gibson |first2=Barry |date=2019 |title=Evidence-based medicine as science |url=https://onlinelibrary.wiley.com/doi/10.1111/jep.13090 |journal=Journal of Evaluation in Clinical Practice |language=en |volume=25 |issue=6 |pages=997–1002 |doi=10.1111/jep.13090 |pmid=30575209 |issn=1356-1294}}</ref>
==Basic statements and the definition of falsifiability==
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{{See also|Historicism#Karl Popper}}
Popper made a clear distinction between the original theory of Marx and what came to be known as Marxism later on.{{sfn|Popper|1995|loc=[https://archive.org/details/in.ernet.dli.2015.77661/page/n101/mode/1up Chap. 15]}} For Popper, the original theory of Marx contained genuine scientific laws. Though they could not make preordained predictions, these laws constrained how changes can occur in society. One of them was that changes in society cannot "be achieved by the use of legal or political means".<ref name="marxismoriginallaw" group=upper-alpha /> In Popper's view, this was both testable and subsequently falsified. "Yet instead of accepting the refutations", Popper wrote, "the followers of Marx re-interpreted both the theory and the evidence in order to make them agree. ... They thus gave a 'conventionalist twist' to the theory; and by this stratagem, they destroyed its much advertised claim to scientific status."<ref name="marxismearlierversionswerefalsifiable" group=upper-alpha /><ref name="thorntonmarxismchangeofstatus" group=upper-alpha /> Popper's attacks were not directed toward Marxism, or Marx's theories, which were falsifiable, but toward Marxists who he considered to have ignored the falsifications which had happened.{{sfn|Smith|2000|p=12}} Popper more fundamentally criticized 'historicism' in the sense of any preordained prediction of history, given what he saw as our right, ability and responsibility to control our own destiny.{{sfn|Smith|2000|p=12}}
===Use in courts of law===
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| title = Word and Flux: The Discrete and the Continuous in Computation, Philosophy, and Psychology. Volume I: From Pythagoras to the Digital Computer, The Intellectual Roots of Symbolic Artificial Intelligence, with a Summary of Volume II Continuous Theories of Knowledge
| url = http://web.eecs.utk.edu/~bmaclenn/WF/WF.pdf
| type = Book in preparation, comments invited
}} * {{Cite web
| url = https://www.iep.utm.edu/sci-ideo/
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| archive-url = https://web.archive.org/web/20191104075438/https://www.iep.utm.edu/sci-ideo/
| archive-date = 4 November 2019
| access-date = 4 November 2019
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| last = Maxwell
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| archive-date = 28 September 2007
}} * {{cite encyclopedia
| first1 = William Edward
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| doi = 10.7551/mitpress/6870.001.0001
| url = https://archive.org/details/towerofbabelevid00penn
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| first = Marcello
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| edition = 2003
| url = https://archive.org/details/objectiveknowled00popp
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| page = 611
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| access-date = 3 May 2021
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| access-date = 5 May 2021
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| archive-url = https://web.archive.org/web/20160818142712/https://books.google.com/books?id=ibePpZ-6lkgC&pg=PA12
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| archive-url = https://web.archive.org/web/20110514101850/http://www.talkorigins.org/faqs/comdesc/default.html
| archive-date = 14 May 2011
| access-date = 21 April 2020
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