Circumscription in a Modal Logic
Fangzhen Lin · 1988
In this paper, we extend circumscription [McCarthy, 80] to a propositional modal logic of knowledge of one agent. Instead of circumscribing a predicate, we circumscribe the knowledge operator "K " in a formula. In order to have a nontrivial circumscription schema, we extend $5 modal logic of knowledge by adding another modality "Val " and a universal quantifier over base sentences (sentences which do not contain modality). In-tuitively, "Val(P) " means that P is a valid formula. It turns out that by circumscribing the knowledge operator in a formula, we completely characterize the maximally ignorant models of the formula (models of the formula where agents have minimal knowledge).