On Modal Logics of Group Belief

Abstract

We overview the existing philosophical accounts of group belief, including both aggregative (or reductionist) approaches reducing collective belief to individual beliefs and non-reductionist approaches ascribing beliefs to the group as a whole. We then provide a modal logic of group belief that follows a non-reductionist approach. We compare our group belief logic with the well-known logic of common belief (which is a logic of collective belief in an aggregative sense) and with the logic of group acceptance that has been recently proposed by some of us. Finally, in the spirit of dynamic epistemic logics we propose an extension of by public announcements.