Propagating epistemic coordination through mutual defaults I
Richmond H. Thomason · 1990
A mutual default is a rule, capable of tolerating exceptions, that is mutually supposed by a group G: i.e., the rule is supposed by all members of the group, is supposed by all members of the group to be supposed by all members of the group, etc. A family of propositional attitudes Bi indexed for i C G (and representing, say, supposition) is coordinated for G if Bi applies to the same propositions for all members i of G, and is commonly supposed by the members of G to do so. This paper is a preliminary exploration of formal postulates that ensure maintenance of coordi-nation of a propositional attitude, representing the common ground of a conversation, in dynamic environments that allow for assertional speech acts. I present results showing that mutually sup-posed rules of conversation provide a mechanism for preserving coordination. If coordination can be assumed, reasoning about propositional attitudes can be greatly simplified, through collapse of iterated operators. I also show how coordination maintenance can be secured, at least in unexceptional cases, when rules of conversation are defeasible; this relaxation of the theory is needed because plausible conversational rules are subject to exceptions.