Universals are a class of mind independent entities, usually contrasted with individuals, postulated to ground and explain relations of qualitative identity and resemblance among individuals. Individuals are said to be similar in virtue of sharing universals. An apple and a ruby are both red, and their common redness results from sharing a universal. If they are both red at the same time, the universal, red, must be in two places at once. This makes universals quite different from individuals, and controversial. Whether universals are in fact required to explain relations of qualitative identity and resemblance among individuals has engaged metaphysicians for two thousand years. Disputants fall into one of three broad camps.
Realists endorse universals. Conceptualists and Nominalists, on the other hand, refuse to accept universals and deny that they are needed. Conceptualists explain similarity among individuals by appealing to general concepts or ideas, things that exist only in minds. Nominalists, in contrast, are content to leave relations of qualitative resemblance brute and ungrounded. Numerous versions of Nominalism have been proposed, some with a great deal of sophistication. Contemporary philosophy has seen the rise of a new form of Nominalism, one that makes use of a special class of individuals, known as tropes. Familiar individuals have many properties, but tropes are single property instances. Whether Trope Nominalism improves on earlier Nominalist theories is the subject of much recent debate. In general, questions surrounding universals touch upon some of the oldest, deepest, and most abstract of philosophical issues.