Concepts as mental representations, where concepts are entities that exist in the mind (mental objects)
Concepts as abilities, where concepts are abilities peculiar to cognitive agents (mental states)
Concepts as Fregean senses (see sense and reference), where concepts are abstract objects, as opposed to mental objects and mental statesConcepts can be organized into a hierarchy, higher levels of which are termed "superordinate" and lower levels termed "subordinate". Additionally, there is the "basic" or "middle" level at which people will most readily categorize a concept. For example, a basic-level concept would be " chair ", with its superordinate, " furniture ", and its subordinate, "easy chair ".
A concept is instantiated (reified) by all of its actual or potential instances, whether these are things in the real world or other ideas.
Concepts are studied as components of human cognition in the cognitive science disciplines of linguistics, psychology and philosophy, where an ongoing debate asks whether all cognition must occur through concepts. Concepts are used as formal tools or models in mathematics, computer science, databases and artificial intelligence where they are sometimes called classes, schema or categories. In informal use the word concept often just means any idea.
Within the framework of the representational theory of mind, the structural position of concepts can be understood as follows: Concepts serve as the building blocks of what are called mental representations (colloquially understood as ideas in the mind ). Mental representations, in turn, are the building blocks of what are called propositional attitudes (colloquially understood as the stances or perspectives we take towards ideas, be it "believing", "doubting", "wondering", "accepting", etc.).
