Universal (metaphysics)

In metaphysics, a universal is something that different specific things have in common, like characteristics or qualities. In simple terms, universals are things that can be shared by many particular objects. For example, imagine there are two green chairs in a room. Both chairs share the quality of being "chairs" and the quality of being "green." These shared qualities are called "universals."

There are three main types of universals: types or kinds (like "mammal"), properties (like "short" or "strong"), and relationships (like "father of" or "next to"). These different categories all represent types of universals.^[1]

Typically, universals are considered abstract, like the idea of "humanity," while particular things are seen as concrete, like a specific person, such as Socrates. But this isn't always the case.^[2] For example, some philosophers think that numbers are particular objects but still abstract. Others, like D. M. Armstrong, believe that universals are concrete, meaning they exist in a real and physical way.

Most philosophers don't think that classes (groups of things) are universals, but some, like John Bigelow, do believe they are.

The problem of universals is a long-standing issue in metaphysics, which deals with whether universals exist. The issue arises because we want to explain why different things, like grass and apples, share the same characteristic, like being green.^[3] The question is: how do we explain this shared characteristic?

Philosophers have many different opinions about universals. For example:

Idealism: "Beauty" only exists in our minds. Platonic extreme realism: "Beauty" exists in a perfect form, independently of our minds or any object. Aristotelian moderate realism or conceptualism: "Beauty" is a characteristic that exists in objects, and our minds recognize it.^[4] Nominalism: There are no universals, only individual things. These views can be grouped into four main categories: extreme realism, nominalism (also known as anti-realism),^[5] moderate realism, and idealism.

A universal can have examples or instances, which are called particulars. For example, "dog" is a universal, and any specific dog is a particular. Similarly, "redness" is a universal, and any particular red object is an example of that universal.

Platonic realism suggests that universals are the meanings behind general terms, such as "sameness" or "beauty." Particular things, like a specific person or object, are not universals. For example, Plato thought that ideas like "circle" or "number" are universals. He saw these universals as the perfect forms, while particular things "participate" in these forms.

Nominalists believe that universals are not real, independent things. They think that universals are either just concepts in our minds or simply names we use to describe things. They argue that properties are individual, specific things rather than universals.

The "ness-ity-hood" principle is a simple way to create names for universals.^[6] By adding the suffix "ness," "ity," or "hood" to a word, you can name a universal. For example, adding "ness" to "left-handed" gives us "left-handedness," which is the name of the universal quality shared by left-handed people.

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• Aristotle, Metaphysics (link)
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• Plato, Phaedo (link)
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• Russell, Bertrand (1912b). "On the Relation of Universals and Particulars" (link)
• Swoyer, Chris (2000). "Properties", The Stanford Encyclopedia of Philosophy, Edward N. Zalta (ed.). (link)
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• Chrysippus – Stanford Encyclopedia of Philosophy
• Chrysippus – Internet Encyclopedia of Philosophy