Modellings for belief change: Base contraction, multiple contraction, and epistemic entrenchment (preliminary report)

Abstract

This paper draws a distinction between the set of explicit beliefs of a reasoner, the “belief base”, and the beliefs that are merely implicit. We study syntax-based belief changes that are governed exclusively by the structure of the belief base. In answering the question whether this kind of belief change can be reconstructed with the help of something like an epistemic entrenchment relation in the sense of Gärdenfors and Makinson [8], we extract several candidate relations from a belief base. The answer to our question is negative, but an approximate solution is possible, and in some cases the agreement is even perfect. Two interpretations of the basic idea of epistemic entrenchment are offered. It is argued that epistemic entrenchment properly understood involves multiple belief changes, i.e., changes by sets of sentences. Since none of our central definitions presupposes the presence of propositional connectives in the object language, the notion of epistemic entrenchment becomes applicable to the style of knowledge representation realized in inheritance networks and truth maintenance systems.