Grice: you mean more than the words you actually say

Paul Grice noticed conversation runs on an unspoken deal: be as informative, honest, relevant and clear as the moment calls for. Say 'some of the cookies are gone' and listeners assume you mean not all, even though 'some' logically allows 'all.' Break these unspoken rules on purpose — answer a question with something irrelevant — and you communicate exactly by breaking them.

— H. P. Grice, Studies in the Way of Words — William James Lectures 1967, published 1989
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