A coherence-based approach to default reasoning

Abstract

In the last 15 years, several default reasoning systems have been proposed to deal with rules having exceptions. Each of these systems has been shown to be either cautious (where some intuitive conclusions do not follow from the default base), or adventurous (some debatable conclusions are inferred). However, the cautiousness and the adventurous aspect of these systems are often due to the incomplete way of describing our knowledge, and that plausible conclusions depend on the meaning (semantics) assigned to propositional symbols. This paper mainly contains two parts. The first part discusses, with simple default bases (where the used symbols have no a priori meaning), which assumptions are assumed when a given conclusion is considered as intuitive. The second part investigates a local approach to deal with default rules of the form “generally, if α then β” having possibly some exceptions. The idea is that when a conflict appears (due to observing exceptional situations), we first localize the sets of pieces of information which are responsible for conflicts. Next, using a new definition of specificity, we attach priorities to default rules inside each conflict. Lastly, three proposals are made to solve conflicts and restore the consistency of the knowledge base. A comparative study with some existing systems is given.