On Belief Revision Based On AGM And DDL

AGM theory,as a milestone in the study of belief change,provides a simpler,more general and more common formal framework for the study of belief change.However,in this formal framework,belief change is discussed on the level of meta-linguistic.In order to study the logical laws and rules that govern our beliefs about belief revision,DDL converts the metalinguistic expression in AGM theory to an object-linguistic expression.This makes belief change theory more convenient when dealing with conditional belief and iterated belief.In order to have a deeper understanding of the different performance of AGM and DDL in the process of belief change,this paper mainly studies AGM and DDL from three aspects: closure,conditional belief and iterated belief revision.The paper introduce Levi’s interpretation about the nature of closure on belief sets in the AGM theory,and reveal why conditional belief is beyond the scope of belief sets in the AGM theory as well as give Rott’s and Levi’s processing methods of characterizing conditional belief.Since conditional belief cannot be characterized in the AGM theory,this paper begins to analyze the different processing methods of iterated belief revision by Darwiche and Nayak.Then on this basis the paper discusses modified KGM system of the DDL processing conditional belief and iterated belief revision as well as onion semantics in terms of AGM theory.And according to the distinction of belief update and belief revision the reason why the iterative belief revision is not completed by AGM is that conditional belief cannot be characterized(belief revision)in the AGM theory,yet DDL can accomplish iterative belief revision because of conditional belief characterized by DDL(belief update).