The Logic of Justified Belief Change, Soft Evidence and Defeasible Knowledge

We present a logic for reasoning about the evidence-based knowledge and beliefs and the evidential dynamics of non-logically-omniscient agents. We do this by adapting key tools and techniques from Dynamic Epistemic Logic, Justification Logic, and Belief Revision so as to provide a lightweight, yet finegrained approach that characterizes well-known epistemic and doxastic attitudes in terms of the evidential reasoning that justifies these attitudes. We then add the dynamic operations of evidence introduction, evidence-based inference, strong acceptance of new evidence (evidential “upgrade”), and irrevocable acceptance of additional evidence (evidential “update”). We exemplify our theory by providing a formal dynamic account of Lehrer’s well-known Gettier-type scenario involving the famous Ferrari and the infamous Messrs. Nogot and Havit.