What a term means has two components: i) the referent of the term -- the class of objects in the world to which the term refers; and ii) the sense of the term, i.e., all of the psychological associations that one has with that term (i.e., the psychological concept related to the term). This second sense of what a term means is referred to as the intension of the term.
Examples of the two components follow. The referent of the term 'cat' is all the cats; the sense of the term is related to your experience of cats, their history, their attributes, etc. A classic example is 'the morning star' and 'the evening star'; both of which refer to the same thing, the planet 'Venus', but the sense of 'morning star' and 'evening star' is not the same.
Intension means that you cannot change terms in a sentence and preserve the truth value of the sentence if they have different intensions. For instance, if you replaced 'the morning star" in a sentence with 'the evening star' the truth value would not be presevered, because the two terms have different intensions.
Other words that are sometimes used to pick out the distinctions between 'extension' and 'intension' are 'denotation' and 'connotation', respectively. Note the following definition by Cohen and Nagel (1993):
A term [an element of a proposition] may be viewed in two ways, either as a class of objects (which may have only one member), or as a set of attributes or characteristics which determine the objects. The first phase or aspect is called the denotation or extension of the term, while the second is called the connotation or intension. The extension of the term 'philosopher' is 'Socrates', 'Plato', 'Thales', and the like; its intension is 'lover of wisdom', 'intelligent', and so on. (31)
The distinctions in the meaning of a term are important to clarify. Without such distinctions, no discussion of meaning in general can begin. If we wish to construct models and theories of human language and thought--and here talk of meaning necessarily enters--we need to make precise those issues and problems we specifically want to address.
- Cohen, M. R. and Nagel, E. (1993). An Introduction To Logic. Indianapolis, Indiana: Hackett Publishing Company.
(Revised March 2010)