A concept is a mental object with the following properties:
- It may be designated by (= be the meaning of) a linguistic expression. It is, however, independent from such linguistic expressions and not identical with any of them.
- It is general: its denotation – if it has any – is a class of objects, not an individual object.
- It is abstract: since there is a variety of objects that may fall (or be subsumed) under a concept, the concept is not specified for those properties that vary among its denotata. As a consequence, it is not instantiated by any particular object; one cannot point to a concept.
- It is a relatively independent unit of thinking. In that, it differs from operations and logical entities (operators) such as junctors and quantifiers, which lack this kind of psychological separability.
- It is dynamic in the sense that it takes part in mental operations. It may be formed by an operation (s. the section on concept formation), it may serve as an operator or as an operand in an operation; it may be changed both in actual thinking and diachronically.
Some examples of concepts are ‘red’, ‘abhor’, ‘plant’, ‘rose’. The quotation marks indicate that what is being represented is a concept, rather than a word.
Some examples of non-concepts are the following:
- red, abhor, plant, rose are not concepts (and instead English words; cf. criterion #1).
- There are no concepts corresponding to Maggie Thatcher or Mount Everest, because they fail on criterion #2.
- There are no concepts corresponding to the English expressions or, it, who, to and many more, because their meanings fail on criterion #4.