intensionalism

Intensionalism is a philosophical position concerning the meaning of linguistic expressions, particularly terms and sentences. It stands in contrast to extensionalism, which focuses on the set of objects to which an expression refers. Intensionalism argues that the meaning of an expression is not solely determined by what it refers to (its extension) but also by its sense or connotation (its intension).

For example, consider the phrases "the morning star" and "the evening star." In terms of extension, both phrases refer to the same object: the planet Venus. An extensionalist view would hold that these phrases have the same meaning because they refer to the same thing. However, an intensionalist would argue that they have different meanings because they convey different modes of presentation or different concepts. One refers to Venus as it appears in the morning, and the other as it appears in the evening. The intension, therefore, captures the concept or property associated with the term.

This distinction is particularly important in the philosophy of language and logic, especially when dealing with modal contexts (e.g., necessity, possibility) and belief contexts. In sentences involving these contexts, replacing a term with another term that has the same extension but a different intension can change the truth value of the sentence. For instance, "It is necessary that the morning star is a planet" is true, while "It is necessary that the morning star is the evening star" is false, even though "the morning star" and "the evening star" refer to the same planet. Intensionalism provides a framework for understanding why such substitutions can alter truth values.