Three Scenarios for the Revision of Epistemic States

This position paper discusses the difficulty of interpreting iterated belief revision in the scope of the existing literature. Axioms of iterated belief revision are often presented as extensions of the AGM axioms, upon receiving a sequence of inputs. More recent inputs are assumed to have priority over less recent ones. We argue that this view of iterated revision is at odds with the claim, made by Gärdenfors and Makinson, that belief revision and non-monotonic reasoning are two sides of the same coin. We lay bare three different paradigms of revision based on specific interpretations of the epistemic entrenchment defining an epistemic state and of the input information. If the epistemic entrenchment stems from default rules, then AGM revision is a matter of changing plausible conclusions when receiving specific information on the problem at hand. In such a paradigm, iterated belief revision makes no sense. If the epistemic entrenchment encodes prior uncertain evidence and the input information is at the same level as the prior information and possibly uncertain, then iterated revision reduces to prioritized merging. A third problem is one of the revision of an epistemic entrenchment by means of another one. In this case, iteration makes sense, and it corresponds to the revision of a conditional knowledge base describing background information by the addition of