The Relationship Between Abduction and Changes in Belief States

Belief revision attempts to model how an agent's belief states change under the innuence of new information. The AGM framework, for example, identiies three types of change: expansion, contraction and revision. But, given an updated belief state (i.e., one that has been expanded, contracted or revised) and the nature of the new information , is it possible to determine or say something about the initial belief state? In this paper we consider contraction and argue that the process of attempting to determine the nature of an initial belief state from a contracted belief state and an epistemic input can be considered one of abduction. We also brieey look at the implications of this in terms of \sphere" semantics.