In metaphysics, a universal is what particular things have in common, namely characteristics or qualities. In other words, universals are repeatable or recurrent entities that can be instantiated or exemplified by many particular things. For example, suppose there are two chairs in a room, each of which is green. These two chairs both share the quality of "chairness", as well as greenness or the quality of being green; in other words, they share a "universal". There are three major kinds of qualities or characteristics: types or kinds (e.g. mammal), properties (e.g. short, strong), and relations (e.g. father of, next to). These are all different types of universals.
Paradigmatically, universals are abstract (e.g. humanity), whereas particulars are concrete (e.g. the personhood of Socrates). However, universals are not necessarily abstract and particulars are not necessarily concrete. For example, one might hold that numbers are particular yet abstract objects. Likewise, some philosophers, such as D. M. Armstrong, consider universals to be concrete.
A universal may have instances, known as its particulars. For example, the type dog (or doghood) is a universal, as are the property red (or redness) and the relation betweenness (or being between). Any particular dog, red thing, or object that is between other things is not a universal, however, but is an instance of a universal. That is, a universal type
A universal may have instances, known as its particulars. For example, the type dog (or doghood) is a universal, as are the property red (or redness) and the relation betweenness (or being between). Any particular dog, red thing, or object that is between other things is not a universal, however, but is an instance of a universal. That is, a universal type (doghood), property (redness), or relation (betweenness) inheres in a particular object (a specific dog, red thing, or object between other things).
Platonic realism holds universals to be the Platonic realism holds universals to be the referents of general terms, such as the abstract, nonphysical, non-mental entities to which words such as "sameness", "circularity", and "beauty" refer. Particulars are the referents of proper names, such as "Phaedo," or of definite descriptions that identify single objects, such as the phrase, "that bed over there". Other metaphysical theories may use the terminology of universals to describe physical entities.
Plato's examples of what we might today call universals included mathematical and geometrical ideas such as a circle and natural numbers as universals. Plato's views on universals did, however, vary across several different discussions. In some cases, Plato spoke as if the perfect circle functioned as the form or blueprint for all copies and for the word definition of circle. In other discussions, Plato describes particulars as "participating" in the associated universal.
Contemporary realists agree with the thesis that universals are multiply-exemplifiable entities. Examples include by D. M. Armstrong, Nicholas Wolterstorff, Reinhardt Grossmann, Michael Loux.
Nominalists hold that universals are not real mind-independent entities but either merely concepts (sometimes called "conceptualism") or merely names. Nominalists typically argue that properties are abstract particulars (like tropes) rather than universals. JP Moreland distinguishes between "extreme" and "moderate" nominalism. Examples of nominalists include the medieval philosophers Roscelin of Compiègne and William of Ockham and contemporary philosophers W. V. O. Quine, Wilfred Sellars, D. C. Williams, and Keith Campbell.