Something is often considered abstract if it does not exist at any particular place and time but instances, or members, of it can exist in many different places and/or times (we say that what is abstract can be multiply instantiated).
If however we just say that what is abstract is what can be instantiated, and that abstraction is simply the movement in the opposite direction to instantiation, we haven't explained everything. That makes 'dog' and 'telephone' abstract ideas, but even small children can recognise a dog or a telephone despite their varying appearances in particular cases. You could say that these concepts are abstractions but are not found to be very abstract in a conceptual sense. We can look at the progression from dog to mammal to animal, and see that animal is more abstract than mammal; but on the other hand mammal is a harder idea to express, certainly in relation to marsupial.
Things are often said to be concrete, that is, not abstract, when they have physical existence or when they occupy space.
Abstract things are sometimes defined as those things that do not exist in reality or exist only as sensory experience, like red. The problem begins to arise here when we try to decide which things are, in fact, real. Is God real, or abstract? Even if real, could God also be abstract? Is the number 2 real? Is goodness real, or only its effects, or is it just an abstract idea created by humans?
Abstraction used in philosophy
Abstraction in philosophy is the (oft-alleged) process, in concept-formation, of recognizing among a number of individuals some common feature, and on that basis forming the concept of that feature. The notion of abstraction is important to understanding some philosophical controversies surrounding empiricism and the problem of universals.
Ontological status of abstract concepts
If we say that properties and relations are, or have being, clearly we mean they have a different sort of being from that which physical objects, like rocks and trees, have. That accounts for the usefulness of the word abstract. We apply it to properties and relations to mark the fact that if they exist, they do not exist in space or time, but that instances of them can exist in many different places.
On the other hand the apple and an individual human being are said to be concrete, and particulars, and individuals.
Confusingly, philosophers sometimes refer to tropes, or property-instances (e.g., the particular redness of this particular apple), as abstract particulars.
Gottlob Frege abstracts abstraction
To be filled with a paragraph or two describing Frege's definition of abstraction.
Reification, also called hypostatization, is usually considered a logical fallacy wherein an abstract concept, such as "society" or "technology" is treated as if it were a concrete thing. It is important to note that reification necessarily occurs linguistically in the English language and many other languages wherein abstract objects are referred to using the same sorts of nouns that signify concrete objects. This can further confuse us about which things are abstract and which concrete, as our language tends to influence us toward reification. To be filled with a sentence or two to give an example of a language in which reification does not occur.
The neurology of abstraction
Some research into the human brain suggests that the left and right hemispheres differ in their handling of abstraction. One side handles collections of examples (eg: examples of a tree) whereas the other handles the concept itself.
Abstract art, abstraction (computer science), abstraction (mathematics), abstract structure, Gottlob Frege, ontology, abstract (summary), model (abstract)
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