Constructive Models for Contraction with Intransitive Plausibility Indifference

Pavlos Peppas, Mary-Anne Williams

Abstract: Plausibility rankings play a central role in modeling Belief Change, and they take different forms depending on the type of belief change under consideration: preorders on possible worlds, epistemic entrenchments, etc. A common feature of all these structures is that plausibility indifference is assumed to be transitive. In a previous article, [7], we argued that this is not always the case, and we introduced new sets of postulates for revision and contraction (weaker variants of the classical AGM postulates), that are liberated from the indifference transitivity assumption. Herein we complete the task by making the necessary adjustments to the epistemic entrenchment and the partial meet models. In particular we lift the indifference transitivity assumption from both these two models, and we establish representation results connecting the weaker models with the weaker postulates for contraction introduced in [7].