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“Metaphysical objects that are not actualized somewhere in space and time, that is, non-particulars such as numbers, properties, relations, propositions, and classes. They stand in contrast to spatio-temporal physical objects.
“Whether these entities actually exist—whether we should ascribe reality to them—is a question of persistent dispute in philosophy. Empiricists and nominalists try to conceive of abstract entities as having merely a linguistic basis. However, if mathematics embodies general truths about the world and has abstract entities as its subject matter, abstract entities would be objects of reference and hence real existents. This is the claim of Platonism and is also a position admitted by Quine’s criterion of ontological commitment. The discussion of abstract entities is related to problems of being, to the problem of universals, also to the theory of meaning.”
— The Blackwell Dictionary of Western Philosophy (Wiley, 2009), 4.