problem of universals

problem of universals, in metaphysics, the question of whether there are universals—“general” things of which particular things are instances or examples or cases—and, if so, what exactly they are and how human knowledge of them is possible. A believer in universals might hold, for example, that, in addition to particular horses, the world contains the species Equus caballus, a general thing of which every horse is an instance (and of which only horses are instances). The problem of universals was a dominant theme in ancient Greek philosophy, in medieval Scholasticism, and in Western philosophy during the modern period (the 17th through the 19th century). Although debates about universals no longer lead to fisticuffs (as they were said to have done among Scholastics), they remain central to contemporary metaphysics.

A strikingly high proportion of the writings of medieval philosophers is directed at disputes about the nature of universalia (Latin: “universals”) and the nature of their relation to the particulars that are their instances. The medieval interest in universals is at least partly to be explained by the respect for philosophical authority characteristic of the Middle Ages and by the fact that the two greatest authorities of ancient Greek philosophy, Plato (428/427–348/347 bce) and Aristotle (384–322 bce), had disagreed about such matters.

According to Plato, when the same general word (e.g., horse, spear, river) applies to different particulars (or to the same particular at different times), it does so by virtue of the fact that those things bear a common relation to a certain form (sometimes called an “idea”)—a supersensible, eternal, and changeless being. If, for example, the word horse applies to each of two particulars, it is only because both of them fall under or “participate” in the form Horse. (And if horse applies to Bucephalus—the famous horse of Alexander the Great—on both Sunday and Monday, it is only because Bucephalus participates in Horse on both Sunday and Monday.) If the application to particular things of general terms like horse were not in some way guided by an understanding of their associated forms, Plato contended, the fact that speakers apply the word to each of two particular things would be a mere accident and would not reflect any common nature among the things so designated—as is the case with the application of the name Heraclitus to each of a dozen or so ancient Greeks.

It seems to be an indisputable consequence of Plato’s theory of forms that the existence of any form does not require that there be a particular thing that participates in it. If, for example, horses were to become extinct, the form Horse, being eternal and changeless, would continue to exist.

Aristotle agreed with Plato on one important point: when a general term is applied to many particular things, its application is guided by knowledge of something that those things have in common. Aristotle denied, however, that what (for example) all horses have in common is participation in a changeless, eternal, independently existing form. In his view, what all horses have in common is something that inheres in each horse—in each of the multiplicity of particular horses.

Plato’s and Aristotle’s contrasting conceptions of universals figured prominently in medieval disputes about universals and their relation to particulars. Four schools of thought emerged from those debates:

1. Platonists, or Platonic realists, affirmed the existence of universalia ante rem (or ante res): universals “before the thing” (or “before things”). That is, they held that general things—the things that account for the fact that a general word applies to many particulars—exist independently of those particulars. According to medieval Platonic realism, before God created any horses, the species horse, or the attribute “being a horse” (or both), already existed. Some medieval Platonists, concerned to avoid the implication that anything might exist independently of God, identified universalia ante rem with ideas in the mind of God.
2. Aristotelians, or Aristotelian realists, affirmed the existence of universals but contended that they were universalia in re (or in rebus): universals “in the thing” (or “in things”). According to Aristotelians, universals exist only as constituents of particulars. The word horse applies to two or more things by virtue of their having a certain universal as a common constituent—the species horse or the attribute “being a horse” (or both). If God had not been pleased to create horses, the Aristotelians maintained, neither the species horse nor the attribute “being a horse” would have existed.
3. Nominalists denied the existence of both universalia ante rem and universalia in re. Reality, they maintained, consists entirely of particulars. The word horse, then, does not refer to a universal, whether before or in things, but simply denotes all horses. Some nominalists did not exactly deny the existence of universals but rather identified them with the general terms whose application Platonists and Aristotelians had invoked universals to explain. (The term nominalism is derived from the Latin word nomina, meaning “names.”) A universal in the Platonic or Aristotelian sense, such nominalists contended, is a mere “puff of sound” (flatus vocis).
4. Finally, conceptualists held that universals are mental entities, confined to individual human minds. Thus, any person’s use of the general term horse is governed by a concept that exists only in that person’s mind. Another person’s use of the word is governed by a numerically distinct concept in that other person’s mind—and those two concepts may well differ in content, with the consequence that the two speakers apply horse to different things. There is no question of a speaker’s associating the “wrong” concept with the word horse—except insofar as that concept leads the speaker to apply the word to objects different from those to which the majority of fellow speakers apply it.

The schools described above are, to a very large extent, abstractions. In practice, it is often difficult to place a given philosopher in any one of them to the complete exclusion of the others. (It is often particularly hard to decide whether a given philosopher should be called a nominalist or a conceptualist.) As a very general rule, it can be said that philosophers writing in Latin in late antiquity tended to be Platonic realists, that philosophers of the high Middle Ages tended to be Aristotelian realists, and that philosophers of the late Middle Ages tended to be nominalists.

Theories of universals received relatively little attention from philosophers of the Renaissance and modern periods. That was particularly true of the empirically minded British philosophers of the 18th century, who generally adopted an uncritical nominalism and who tended to regard any attempt at serious discussion of the nature of universals as so much Scholastic blather.

In the 20th century there was a remarkable resurgence of interest in theories of universals. There were, on the one hand, a significant number of philosophers who defended the thesis that an appeal to Aristotelian universals explains various features of the world (e.g., that a green book and a green apple have something in common). And there was, on the other hand, a revival of Platonic realism, in large part a consequence of the realism about mathematical objects advocated by Bertrand Russell (1872–1970) and the philosophy of logic of W.V.O. Quine (1908–2000)—particularly Quine’s views on quantification and “ontological commitment.”