intensionality failure of extensionality. A linguistic context is extensional if and only if the extension of the expression obtained by placing any subexpression in that context is the same as the extension of the expression obtained by placing in that context any subexpression with the same extension as the first subexpression. Modal, intentional, and direct quotational contexts are main instances of intensional contexts. Take, e.g., sentential contexts. The extension of a sentence is its truth or falsity (truth-value). The extension of a definite description is what it is true of: ‘the husband of Xanthippe’ and ‘the teacher of Plato’ have the same extension, for they are true of the same man, Socrates. Given this, it is easy to see that ‘Necessarily, . . . was married to Xanthippe’ is intensional, for ‘Necessarily, the husband of Xanthippe was married to Xanthippe’ is true, but ‘Necessarily, the teacher of Plato was married to Xanthippe’ is not. Other modal terms that generate intensional contexts include ‘possibly’, ‘impossibly’, ‘essentially’, ‘contingently’, etc. Assume that Smith has heard of Xanthippe but not Plato. ‘Smith believes that . . . was married to Xanthippe’ is intensional, for ‘Smith believes that the husband of Xanthippe was married to Xanthippe’ is true, but ‘Smith believes that the teacher of Plato was married to Xanthippe’ is not. Other intentional verbs that generate intensional contexts include ‘know’, ‘doubt’, ‘wonder’, ‘fear’, ‘intend’, ‘state’, and ‘want’. ‘The fourth word in ‘. . . ‘ has nine letters’ is intensional, for ‘The fourth word in ‘the husband of Xanthippe’ has nine letters’ is true but ‘the fourth word in ‘the teacher of Plato’ has nine letters’ is not. See also EXTENSIONALISM , MEANING , QUANTIFYING IN , REFERENTIALLY TRANSPAREN. T.Y.