universal, in philosophy, an entity used in a certain type of metaphysical explanation of what it is for things to share a feature, attribute, or quality or to fall under the same type or natural kind. A pair of things resembling each other in any of these ways may be said to have (or to “exemplify”) a common property. If a rose and a fire truck are the same colour, for example, they both exemplify redness, or the property of being red. Realists take this way of talking about universals to be strictly and literally true: the property shared—redness—is a third entity, distinct from both the rose and the truck. The two things resemble each other in virtue of standing in the same relation (“exemplification”) to this third entity, which is called a “universal” because it extends over, or is located in, many distinct things. Nominalists, on the other hand, reject universals, claiming that there is no need to posit an extra, rather strange entity—the universal “redness”—simply to account for the fact that roses and fire trucks resemble one another.

The problem of universals—whether there are any and, if so, what exactly they are—was a dominant theme in ancient Greek philosophy, in medieval Scholasticism, and in Western philosophy during the modern period (the 17th through the 19th centuries). Although debates about universals no longer lead to fisticuffs (as they were said to do among the Scholastics), they remain central to contemporary metaphysics. Realists are still opposed by nominalists, and realists themselves are still sharply divided between those who adhere to something like Plato’s conception of universals and those who favour Aristotle’s. Realists also remain divided between those who posit a plenitude of universals and those who accept very few. This division in turn reflects a fundamental disagreement among realists over why one should believe in universals in the first place.

Platonic and Aristotelian realism

According to a traditional interpretation of the metaphysics of Plato’s middle dialogues, Plato maintained that exemplifying a property is a matter of imperfectly copying an entity he called a form, which itself is a perfect or pure instance of the property in question. Several things are red or beautiful, for example, in virtue of their resembling the ideal form of the Red or the Beautiful. Plato’s forms are abstract or transcendent, occupying a realm completely outside space and time. They cannot affect or be affected by any object or event in the physical universe.

Few philosophers now believe in such a “Platonic heaven,” at least as Plato originally conceived it; the “copying” theory of exemplification is generally rejected. Nevertheless, many modern and contemporary philosophers, including Gottlob Frege, the early Bertrand Russell, Alonzo Church, and George Bealer are properly called “Platonic” realists because they believed in universals that are abstract or transcendent and that do not depend upon the existence of their instances.

Aristotle denied that exemplifying a universal is anything like copying it. He parted company with all Platonic realists by affirming that: (1) the properties of material things are “immanent”—i.e., “in” the things that exemplify them, in a nearly literal, spatial way; and (2) properties do not exist independently of the things that exemplify them. Both of these ideas survived in some contemporary theories. Thus, the entities that Alfred North Whitehead called “objects” seem to be universals weaving their way through space-time, numerically the same wherever or whenever they appear. Universals are also immanent according to defenders of the so-called “bundle” theory—philosophers such as David Hume and the later Russell, who said that individuals are just bundles of universals. An individual stop sign, for example, consists of the universals eight-sidedness, redness, hardness, and so on. Because the sign is spatially and temporally located and contains nothing but universals, universals themselves must be in space and time—they cannot be there merely “by courtesy,” in virtue of being exemplified by something that is really in space and time. In the theory defended by the contemporary Australian philosopher David Armstrong, universals are perhaps not quite as immanent as they are according to the bundle theorists, but they nevertheless obey an Aristotelian “principle of instantiation,” insofar as no universal can exist without instances.

Medieval and early-modern nominalism

The problem of universals was arguably the central theme of medieval Western philosophy. Just before the medieval period, St. Augustine defended a version of Platonism, identifying Platonic forms with exemplars timelessly existing in the mind of God. Although many medieval philosophers were Aristotelian realists of one sort or another, a few developed varieties of nominalism. William of Ockham, for example, claimed that things “share features in common” in virtue of the fact that objective relations of resemblance hold among them. But he denied that the holding of such relations requires that there be anything literally the same within the things themselves. Ockham explained the human ability to think and talk in general terms by appealing to mental entities, or concepts, which serve as “natural signs” of the many things to which they apply.

Ockham’s conceptualism won few converts among medieval philosophers. But conceptualism of one sort or another, combined with nominalism, was central to the philosophies of the 17th- and 18th-century British empiricists John Locke, George Berkeley, and Hume.

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It should be noted that there is much inconsistency in the application of the terms “conceptualism” and “nominalism.” In this article “nominalist” is used as a name for all opponents of realism, some of whom have typically been called “conceptualists.” It would be least misleading to reserve the term “conceptualism” for certain theories about the ability to think and talk in general terms—theories that are not, strictly speaking, in competition with realism or nominalism but that, in practice, usually accompany the latter.