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 (singular 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 a type of thing, ie, an idea , orabstraction .  The term ‘abstract object’ is said to be coined by Willard Van Orman Quine .  The study of abstract objects is called abstract object theory .
The type-token distinction identified 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 understood from the following points of view:
|Tennis||A tennis match|
|Redness||Red light reflected off an apple and hitting your eyes|
|Justice||A just action|
|Humanity (the property of being human)||Human population (the set of all humans)|
Abstract objects have a lot of interest in philosophers because they have problems for popular theories. In ontology , abstract objects are considered problematic for physicalism and some forms of naturalism . Historically, the most important ontological dispute on the subject of the 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.
Some, Such As Edward Zalta and arguably, Plato In His Theory of Forms , abstract objects-have Held That Constitute defining the subject matter of metaphysics or philosophical inquiry Broadly more. To the extent that this theory is independent of empirical research, and to the extent that empirical questions do not inform questions about the subject, it would be particularly important to answer these questions.
In modern philosophy, the distinction between abstract and concrete was explored by Immanuel Kant  and GWF Hegel . 
Gottlob Frege said that these objects are of a third realm ,  different from the external world or of internal consciousness. 
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 can not act on other objects. One problem for this view is that it is not clear exactly what is causal power. For a more detailed exploration of the abstract-concrete distinction, follow the link to the Stanford Encyclopediaarticle. 
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|
|Dense things sink.||It will sink if its density is greater than the density of the fluid.|
|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 quasi-abstract about the development of a third category of objects. 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 of the duality of the concrete and the abstract has led to a large category of considered incompatible.  Specially, the ability to have temporal location, and 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. 
- Abstract object theory
- Abstract particulars
- Abstract structure
- Conceptual framework
- Non-physical entity
- Object (philosophy)
- Object of the mind
- Philosophy of mathematics
- Platonic form
- Universal (metaphysics)
- Jump up^ Abrams, Meyer Howard; Harpham, Geoffrey Galt (2011). A Glossary of Literary Terms . ISBN 0495898023 . Retrieved 18 September 2012 .
- Jump up^ Armstrong, DM (2010). Sketch for a systematic metaphysics . Oxford: Oxford University Press. p. 2. ISBN 9780199655915 .
- Jump up^ Carr, Philip (2012) “The Philosophy of Phonology” inPhilosophy of Linguistics(Kemp, Fernando, Asher), Elsevier, p. 404
- Jump up^ KrV A51 / B75-6. See also: Edward Willatt,Kant, Deleuze and Architectonics, Continuum, 2010 p. 17: “Kant argues that cognition can only come about as a result of the union of the abstract work of understanding and the concrete input of sensation.”
- Jump up^ Georg Wilhelm Friedrich Hegel: The Science of Logic , Cambridge University Press, 2010, p. 609. See also: Richard Dien Winfield,Hegel’s Science of Logic: A Critical Rethinking in Thirty Readings, Rowman & Littlefield Publishers, 2012, p. 265.
- Jump up^ Gottlob Frege, “Der Gedanke, Eine logische Untersuchung,” in:Beiträge zur Philosophie des deutschen Idealismus1 (1918/19), pp. 58-77; esp. p. 69.
- Jump up^ Rosen, Gideon (1 January 2014). “Abstract Objects” . The Stanford Encyclopedia of Philosophy . Metaphysics Research Lab, Stanford University . Retrieved 1 January 2017 .
- Jump up^ Rosen, Gideon. “Abstract Objects” . In Zalta, Edward N. Stanford Encyclopedia of Philosophy .
- Jump up^ B. Smith (2008), “Searle and De Soto: The New Ontology of the Social World.” InThe Mystery of Capital and the Construction of Social Reality. Open Court.
- Jump up^ EH Robinson,”A Theory of Social Agentivity and Its Integration into the Descriptive Ontology for Linguistic and Cognitive Engineering”,International Semantic Web and Information Systems Journal is 7(4) (2011) pp. 62-86.
- Jump up^ EH Robinson (2014), “A Documentary Theory of States and Their Existence as Quasi-Abstract Entities,”Geopolitics 19(3), pp. 1-29.