Abstract objects

Abstract and concrete (German: abstrakt; konkret) are classifications that denote whether a term describes an object with a physical referent or one with no physical referents. They are most commonly used in philosophy and semantics. Abstract objects are sometimes called abstracta (sing. abstractum) and concrete objects are sometimes called (sing. concretum). An abstract object is an object which does not exist at any particular time or place, but rather exists as a type of thing, i.e., an idea, or abstraction. The term 'abstract object' is said to have been coined by Willard Van Orman Quine. The study of abstract objects is called abstract object theory.

The type–token distinction identifies physical objects that are tokens of a particular type of thing. The "type" that it is a part of, is in itself an abstract object. The abstract-concrete distinction is often introduced and initially understood in terms of paradigmatic examples of objects of each kind:

Abstract objects have often garnered the interest of philosophers because they raise problems for popular theories. In ontology, abstract objects are considered problematic for physicalism and some forms of naturalism. Historically, the most important ontological dispute about abstract objects has been the problem of universals. In epistemology, abstract objects are considered problematic for empiricism. If abstracta lack causal powers or spatial location, how do we know about them? It is hard to say how they can affect our sensory experiences, and yet we seem to agree on a wide range of claims about them.

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