In philosophy and the arts, a fundamental distinction is between things that are abstract and things that are concrete. While there is no general consensus as to how to precisely define the two, examples include that things like numbers, sets, and ideas are abstract objects, while plants, dogs, and planets are concrete objects.[1] Popular suggestions for a definition include that the distinction between concreteness versus abstractness is, respectively: between (1) existence inside versus outside space-time; (2) having causes and effects versus not; 3) being related, in metaphysics, to particulars versus universals; and (4) belonging to either the physical versus the mental realm (or the mental-and-physical realm versus neither).[2][3][4] Another view is that it is the distinction between contingent existence versus necessary existence; however, philosophers differ on which type of existence here defines abstractness, as opposed to concreteness. Despite this diversity of views, there is broad agreement concerning most objects as to whether they are abstract or concrete,[1] such that most interpretations agree, for example, that rocks are concrete objects while numbers are abstract objects.
Abstract objects are most commonly used in philosophy, particularly metaphysics, and semantics. They are sometimes called abstracta in contrast to concreta. The term abstract object is said to have been coined by Willard Van Orman Quine.[5] Abstract object theory is a discipline that studies the nature and role of abstract objects. It holds that properties can be related to objects in two ways: through exemplification and through encoding. Concrete objects exemplify their properties while abstract objects merely encode them. This approach is also known as the dual copula strategy.[6]