ON THE LOGIC OF ITERATED BELIEF REVISION

We show in this paper that the AGM postulates are too weak to ensure t he rational preservation of conditional beliefs during belief revision , thus permitting improper responses to sequences of observations. We remedy this weakness by proposing four additional postulates, which ar e sound relative to a qualitative version of probabilistic conditionin g. Contrary to the AGM framework, the proposed postulates characterize belief revision as a process which may depend on elements of an epist emic state that are not necessarily captured by a belief set. We also show that a simple modification to the AGM framework can allow belief revision to be a function of epistemic states. We establish a model-ba sed representation theorem which characterizes the proposed postulates and constrains, in turn, the way in which entrenchment orderings may be transformed under iterated belief revision.