A typical use of any of the three sentences below would take for granted that Mary was a communist.(1) a. John knew Mary was a communist. b. John didn't know Mary was a communist. c. If John knew that Mary was a communist, he'd have fired her.

These sorts of observations stretch back, in their contemporary form, to the discussion of definite descriptions in Strawson (1956). On the Strawsonian style of analysis? which was not explicit but very suggestive? these judgments are explained by the fact that" jc knows _P" is an expression that requires for its meaningfulness the truth of P. This is one possible explanation of this phenomenon but not the only one, and I will take the subject o? presupposition to be the phenomenon illustrated by (1), without tying the term to any particular style of explanation. When I talk o? presuppositions, I refer to the propositions that utterances of certain sentences take for granted, in the …