Abstract and concrete
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Abstract and concrete 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 concreta (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:
|Tennis||A tennis game|
|Redness||The red coloring of an apple|
|Justice||A just action|
|Humanity (the property of being human)||Humanity (the human race)|
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. Some, such as Edward Zalta and arguably, Plato in his Theory of Forms, have held that abstract objects constitute the defining subject matter of metaphysics or philosophical inquiry more broadly. To the extent that philosophy is independent of empirical research, and to the extent that empirical questions do not inform questions about abstracta, philosophy would seem especially suited to answering these latter questions.
Abstract objects and causality
Another popular proposal for drawing the abstract-concrete distinction contends that an object is abstract if it lacks any causal powers. A causal power has the ability to affect something causally. Thus, the empty set is abstract because it cannot act on other objects. One problem for this view is that it is not clear exactly what it is to have a causal power. For a more detailed exploration of the abstract-concrete distinction, follow the link below to the Stanford Encyclopedia article.
Concrete and abstract thinking
Jean Piaget uses the terms "concrete" and "formal" to describe the different types of learning. Concrete thinking involves facts and descriptions about everyday, tangible objects, while abstract (formal operational) thinking involves a mental process.
|Concrete idea||Abstract idea|
|Heavy things sink.||It will sink if its density is greater than the density of the liquid.|
|You breathe in oxygen and breathe out carbon dioxide.||Gas exchange takes place between the air in the alveoli and the blood.|
|Plants get water through their roots.||Water diffuses through the cell membrane of the root hair cells.|
Recently, there has been some philosophical interest in the development of a third category of objects known as the quasi-abstract. Quasi-abstract objects have drawn particular attention in the area of social ontology and documentality. It has been argued that the over-adherence to the platonist duality of the concrete and the abstract has lead to a large category of social objects having been overlooked or rejected as nonexisting because they exhibit characteristics which the traditional duablity between the concrete and the abstract has regarded as incompatible.  Specially, the ability to have temporal location, but not spatial location, and have causal agency (if only by acting through representatives ). These characteristics are exhibited by a number of social objects, including states of the international legal system. 
- Abrams, Meyer Howard; Harpham, Geoffrey Galt (2011). A Glossary of Literary Terms. ISBN 0495898023. Retrieved 18 September 2012.
- Armstrong, D.M. (2010). Sketch for a systematic metaphysics. Oxford: Oxford University Press. p. 2. ISBN 9780199655915.
- Abstract Objects entry by Gideon Rosen in the Stanford Encyclopedia of Philosophy
- B. Smith, (2008) 'Searle and De Soto: The New Ontology of the Social World”. In The Mystery of Capital and the Construction of Social Reality. Open Court.
- E. H. Robinson, ‘A Theory of Social Agentivity and Its Integration into the Descriptive Ontology for Linguistic and Cognitive Engineering’, International Journal on Semantic Web and Information Systems 7/4 (2011) pp. 62–86.
- E.H. Robinson (2014), “A Documentary Theory of States and Their Existence as Quasi-Abstract Entities,” Geopolitics 00, pp. 1-29.
- Abstract and concrete entry in the Stanford Encyclopedia of Philosophy
- Abstract and concrete at PhilPapers
- Nominalism, Realism, Conceptualism,Interface, from The Catholic Encyclopedia
- Abstract vs. Concrete in Writing, from Writing for Results