A foundationalist view of the AGM theory of belief change

We describe an alternative interpretation of the AGM postulates for belief change (Alchourrón et al., 1985; Gärdenfors, 1988) in a foundational framework of epistemic states suggested by Bochman (1997). Though foundational contractions constitute a new kind of belief contractions, incompatible with AGM postulates, a broad class of preferential AGM contractions is shown to be revisionequivalent to foundational contractions and can actually be produced using a modification of the contraction operation on epistemic states. It is shown also that revisions of epistemic states determine a natural class of preferential AGM revisions, and that all the AGM postulates for revision can be satisfied by imposing reasonable restrictions on underlying epistemic states. As a result, the foundational theory of epistemic states is shown to provide a unification of the main approaches to belief change on a foundationalist basis.  2000 Elsevier Science B.V. All rights reserved.