Basic entrenchment (Q1871132)

A long-standing puzzle about the use of epistemic entrenchment in the semantics of AGM belief change is that while it gives an elegant characterization of the `extended postulates' for contraction and revision, it has resisted attempts to adapt it to characterize what AGM called the `basic postulates'. In this respect, it is quite unlike the partial meet semantics or safe contraction, both of which admit such modularity in a natural way. The paper under review finally solves this problem, and with elegance. At the same time, the author's approach also yields entrenchment-style characterizations of various interesting subsets of the basic postulates.