2 Center for Logic and Philosophy of Science

We present a logical setting that incorporates a belief-revision mecha-nism within Dynamic-Epistemic logic. As the “static ” basis for belief revision, we use epistemic plausibility models, together with a modal language based on two epistemic operators: a “knowledge ” modality K (the standard S5, fully introspective, notion), and a “safe belief” modality 2 (“weak”, non-negatively-introspective, notion, capturing a version of Lehrer’s “indefeasible knowledge”). To deal with “dy-namic ” belief revision, we introduce action plausibility models, repre-senting various types of “doxastic events”. Action models “act ” on state models via a modified update product operation: the “Action-Priority ” Update. This is the natural dynamic generalization of AGM revision, giving priority to the incoming information (i.e., to “ac-tions”) over prior beliefs. We completely axiomatize this logic, and show how our update mechanism can “simulate”, in a uniform man-ner, many different belief-revision policies.