Nominalism is a metaphysical view in philosophy according to which general or abstract terms and predicates exist, while universals or abstract objects, which are sometimes thought to correspond to these terms, do not exist. Thus, there are at least two main versions of nominalism. One version denies the existence of universals—things that can be instantiated or exemplified by many particular things (e.g. strength, humanity). The other version specifically denies the existence of abstract objects—objects that do not exist in space and time.
However, these two versions of nominalism basically collapse into one if one believes that all universals are abstract objects. Most nominalists have held that only physical particulars in space and time are real, and that universals exist only post res, that is, subsequent to particular things. However, some versions of nominalism hold that some particulars are abstract entities (e.g. numbers), while others are concrete entities—entities that do exist in space and time (e.g. tables, chairs).
Nominalism is primarily a position on the problem of universals, which dates back at least to Plato, and is opposed to realism—the view that universals do exist over and above particulars. However, the name "nominalism" emerged out of debates in medieval philosophy with Roscellinus. As John Stuart Mill once wrote, the early versions of nominalism were that "there is nothing general except names", hence the prefix "nomin-". This, however, is a more dated use of the term that is now considered to be a specific version of what is now called "nominalism".
History of nominalism
Plato was perhaps the first writer in Western philosophy to clearly distinguish the Nominalist position from a non-Nominalist one, the latter of which he plainly endorsed:
...We customarily hypothesize a single form in connection with each of the many things to which we apply the same name. ... For example, there are many beds and tables. ... But there are only two forms of such furniture, one of the bed and one of the table. (Republic 596a-b, trans. Grube)
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