Intension and extension
The intension of a concept consists of the qualities or properties which go to make up the concept. The extension of a concept consists of the things which fall under the concept; or, according to another definition, the extension of a concept consists of the concepts which are subsumed under it (determine subclasses). This is the old distinction between intension and extension, and coincides approximately with the distinction between a monadic proposittonal function (q. v.) in intension and a class (q. v.). The words intension and extension are also used in connection with a number of distinctions related or analogous to this one, the adjective extensional being applied to notions or points of view which in some respect confine attention to truth-values of propositions as opposed to meanings constituting propositions. In the case of (interpreted) calculi of propositions or propositional functions, the adjective intensional may mean that account is taken of modality, extensional that all functions of propositions which appear are truth-functions. The extreme of the extensional point of view does away with propositions altogether and retains only truth-values in their place. — A.C.
The Port-Royal Logic, translated by T. S. Baynes (see Introduction by the translator).
Lewis and Langford, Symbolic Logic, New York and London, 1952. R. Carnap, The Logical Syntax of Language, New York and London, 1937.