The process of forming a general concept by omitting every distinguishing feature from our notions of some collection of particular things; thus, substantively, an abstraction is the concept or idea that results from this process. Introduced by Peter Abelard as part of his solution to the problem of universals, abstraction became crucial for other nominalistic explanations, including Locke's account of our use of general terms. Thus, for example, the idea of "green" could in principle be derived by abstracting from one's specific experiences of a summer lawn, the leaves of trees, and emeralds. Berkeley, on the other hand, argued that abstract ideas in this sense are impossible because every sensible idea has only particular content. In the more recent work of Frege, Quine, and Kripke, efforts to understand the status of abstract ideas focus on the proper analysis of general terms in language.

Recommended Reading: Berkeley on Abstraction and Abstract Ideas, ed. by Willis Doney (Garland, 1993) and Roger Teichmann, Abstract Entities (St. Martin's, 1992).

Also see SEP, and Andrew Brook.