Conservative belief change

A standard assumption underlying traditional accounts of belief change is the principle of minimal change, that an agent's belief state should be modified minimally to incorporate new information. In this paper we introduce a novel account of belief change in which the agent's belief state is modified minimally to incorporate exactly the new information. Thus a revision by pΛq will result in a new belief state in which pΛq is believed, but a stronger proposition (such as p Λ q) is not, regardless of the initial form of the belief state. This form of belief change is termed conservative belief change and corresponds to a Gricean interpretation of the input formula. We investigate belief revision in this framework, and provide a representation result between a set of postulates characterising this form of belief change and a construction in terms of systems of spheres. This approach is extended to that of belief revision with respect to a specified context. Last, we show how this approach resolves a longstanding problem in belief revision.