Common Priors For Like-Minded Agents

Two agents are like-minded when their beliefs are equal once conditioned on knowledge of both of their types. Assuming the existence of an outside observer that is commonly known to be likeminded and uninformative about the insiders, we derive the existence of a common prior among the insiders, with the outsiders beliefs (appropriately conditioned) serving as the common prior. A key advantage of like-mindedness is its fully local definition, which allows to distinction between consistency of agent’s actual beliefs and of beliefs they merely view as possible. By later including agents’ “epistemic attitudes” among the primitives, we derive like-mindedness from reasonableness judgments about each others attitudes. In this richer framework, one can model alternative conceptions of intersubjective rationality as constraints on such reasonableness judgements.