The State of the Art
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This chapter offers a brief presentation of major models of belief change that have been developed in previous research. It begins by introducing the basic features shared by most such models: Belief states are represented by logically closed sets of sentences, commonly called belief sets. Changes take the form of either introducing a new sentence into the belief set while retaining consistency (revision) or removing a sentence from it without adding anything else (contraction). The standard model for performing such changes, the so-called AGM model, is introduced along with the axioms used to characterize it. Two of the most important equivalent formulations of the AGM model, possible world models and epistemic entrenchment, are introduced. A couple of other important approaches are also briefly mentioned.