Contraction core for horn belief change: preliminary report

Abstract

In this paper the authors continue recent investigations into belief change for Horn logic. The main contribution is a result which shows that the construction method for Horn contraction for belief sets based on infraremainder sets, as recently proposed by Booth et al., corresponds exactly to Hansson’s classical kernel contraction for belief sets, when restricted to Horn logic. This result is obtained via a detour through Horn contraction for belief bases during which we prove that kernel contraction for Horn belief bases produces precisely the same results as the belief base version of the Booth et al. construction method. The use of belief bases to obtain the result provides evidence for the conjecture that Horn belief change is best viewed as a “hybrid” version of belief set change and belief base change. One of the consequences of the link with base contraction is the provision of a more elegant representation result for Horn contraction for belief sets in which a version of the Core-retainment postulate features. The paper focuses on Delgrande’s entailment-based contraction (e-contraction), but we also mention similar results for inconsistency-based contraction (i-contraction) and package contraction (p-contraction).