Alternative foundations for Reiter's default logic

We introduce an alternative conceptual basis for default reasoning in Reite r's default logic. In fact, most formal or computational treatments of defa ult logic suffer from the necessity of exhaustive consistency checks with r espect to the finally resulting set of conclusions; often this so-called ex tension is just about being constructed. On the theoretical side, this exha ustive approach is reflected by the usual fixed-point characterizations of extensions. Our goal is to reduce such global considerations to local and s trictly necessary ones. For this purpose, we develop various techniques and instruments that draw on an analysis of interaction patterns between defau lt rules, embodied by their mutual blocking behavior. These formal tools pr ovide us with alternative means for addressing a variety of questions in de fault logic. We demonstrate the utility of our approach by applying it to t hree traditional problems. First, we obtain a range of criteria guaranteein g the existence and non-existence of extensions. Second, we get alternative characterizations of extensions that avoid fixed-point conditions. Finally , we furnish a formal account of default proofs that was up to now neglecte d in the literature. (C) 2000 Elsevier Science B.V. All rights reserved.