‘On Being Informed’: Update Logics for Knowledge States

Abstract

Statements convey information, by modifying knowledge states of hearers and speakers. This dynamic aspect of communication goes beyond the usual role of logic as a provider of static ‘truth conditions’. But it can be modelled rather nicely in so-called ‘update logics’, which have been developed since the 1980s. These systems provide a fresh look at standard logic, letting the usual models undergo suitable changes as agents absorb the content of successive utterances or messages. This lecture is a brief Whig history of update logics, with an emphasis on many-agent epistemic languages. We discuss straight update, questions and answers, and the delightful complexities of communication under various constraints. We hope to convey the attraction of giving a dynamic twist to wellknown things, such as simple modal models, or basic epistemic formulas.