Abstract entities such as numbers, propositions, and universals are usually contrasted with concrete entities such as tables, tadpoles, and quasars. While concrete entities are typically held to exist contingently, stand in causal relations, and occupy space and time, abstract entities are usually (though controversially) held to be necessary existents that are causally inert and "outside" of space and time. According to platonists, there are at least some abstract entities, but, given the peculiar metaphysical status of abstract entities, concerns arise about how we might come to know or refer to them. Further controversies surround the nature of the abstract-concrete distinction, the essential features of abstract entities, and the plausibility of nominalism, according to which there are no abstract entities.
|Key works||The literature on abstract entities is vast and closely connected to debates in the philosophy of mathematics, philosophy of science, philosophy of language, and epistemology. Burgess & Rosen 1997 is the leading survey of nominalist views regarding mathematical entities and includes an excellent survey of the nominalist-platonist debate. Two papers by Paul Benacerraf—Benacerraf 1973 and Benacerraf 1965—are hugely influential. Notable recent work regarding nominalist options in the philosophy of mathematics and other domains include Field 1980, Azzouni 2004, Leng 2010, and Hofweber 2016. Guides to the specialist literature regarding specific kinds of abstract entities can be found in other entries, but some useful starting points are as follows. On possible worlds, see Divers 2002. On types, see Wetzel 2009. On propositions, see King et al 2014. On fictional entities, see Thomasson 1998.|
|Introductions||Rosen 2008 and Szabó 2003 are useful survey articles of the issues raised by abstract entities. There are also a host of good entries in the Stanford Encyclopedia of Philosophy on topics related to abstract entities. Balaguer 2008, Linnebo 2009, and Rodriguez-Pereyra 2008 are excellent places to start. Armstrong 1989 is a leading introduction to the metaphysics of properties, which are usually held to be abstract entities. MacBride 1999 and MacBride 2003 survey fictionalist and neo-Fregean approaches to abstract mathematical entities. General introductions to metaphysics with useful sections on abstract entities include Jubien 1997 and Loux & Crisp 1997. Cowling 2017 is a fairly comprehensive and accessible discussion of the metaphysics of abstract entities.|
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