Three scenarios for the revision of epistemic states
Résumé
This position paper discusses the difficulty of interpreting the iterated belief revision problem. Axioms of iterated belief revision are often presented as extensions of the AGM axioms, upon receiving a sequence of inputs, likely to alter not only the belief set, but also the epistemic entrenchment relation underlying the revision operator. Iterated belief revision presupposes that more recent inputs have priority over less recent ones. We argue that this view of iterated revision is at odds with the suggestion of Gärdenfors and Makinson, that belief revision and non-monotonic reasoning are two sides of the same coin. It is not clear that non-monotonic reasoning modifies the ranking of possible worlds implicit in default rules. We lay bare three different paradigms of revision based on specific interpretations of the epistemic entrenchment implicitly at work and of the input information. If the epistemic entrenchment stems from default rules and the input is a specific piece of evidence, then AGM revision is a matter of changing plausible conclusions, and iterated revision makes no sense. However, if the epistemic entrenchment encodes uncertain factual evidence and the input information as well, then iterated revision reduces to prioritized merging. A third problem where iteration makes sense corresponds to the revision, by the addition of new default rules, of a conditional knowledge base describing background information. The three scenarios are compared with similar problems in the framework of probabilistic reasoning