Pruning multi-extensions via exceptions

Abstract

The main contribution of this paper is a method for pruning multi-extensions of a defeasible theory by using the exceptions to order the defeasible fonnulae. We construct a defeasible logic -- DEFEASIBLE WGIC WrrH EXCEFrIONS FIRST(DLEF) -- in which extensions are buílt taking into account the order on the defeasible fonnu1ae induced by the exceptions. This device prompts DLEF as a powerful tool to formalize common sense reasoning. It is on the formalization of the frame problem that we best evaluate the original features of DLEF. DLEF allows the formalization of the persistence axiom in the temporal projection problem in a stepwise way. That is, the persistence axiom is applied locally after every action is performed. Thus, if no exception to sorne properties is present while an action is performed the persistence axiom is used to conclude that those properties will remain unaltered in the resulting situation. Therefore, no property at the present is changed just for the sake of not changing sorne other properties in the future. The only reason for changes in properties are explicit changes provoked by the action being perfonned at the moment.