One of the major intellectual debates of the Middle Ages concerned the problem of universals. The problem has its source in Plato and Aristotle, but medieval philosophers such as Boethius, Abelard, Aquinas, Scotus, and Ockham discussed it with a degree of precision and sophistication that is perhaps unrivaled. It is sometimes noted that there is not one problem here but several, touching on metaphysical, epistemological, and logical questions. The main problem, however, is metaphysical and can be stated as follows: some things in the world resemble each other. Many things are, for example, red, six feet tall, virtuous, and/or human. If two or more things are similar in one of these respects, or in some other respect, in virtue of what are they similar? Is there some entity that they literally share in common – namely, the universal (such as Redness)? If so, what status does this universal have? Does it exist in particulars, or is it distinct and separable from them? Broadly speaking, “realists” are philosophers who affirm the existence of universals and confer some ontological status upon them apart from the mind, whereas “conceptualists” and “nominalists” – for reasons of ontological parsimony – reduce them to ideas, names, or even the spoken word. In the Middle Ages, the debate about universals centered on Aristotle's five “predicables” – genus, species, differentia, property, and accident – that had been treated in an influential work by Porphyry, a third-century Neoplatonist. What, for example, is the ontological status of the species man, or of rationality, which constitutes the differentia that distinguishes man from other species within the genus animal? But for philosophers before and after this period, the problem of universals has always had a much wider application. Plato, for example, was a realist who conceived of Beauty, Truth, and mathematical objects as separately existing universals.

Most early modern philosophers write as if the problem of universals has been solved and typically give the issue only passing attention.