Pluralistic evaluation of belief plausibility and its application to nonmonotonic reasoning

Abstract

A pluralistic evaluation of belief plausibility is introduced by extending the notion of plausibility index introduced by Rescher. The properties of the extended plausibility are analyzed by comparing its properties with those of probability theoretic indexing and Rescher's indexing of beliefs. Also it is shown that the new indexing method can be effectively utilized in truth maintenance of beliefs in complex and dynamically changing situations. First, the reason we focus our attention on Rescher's plausibility index of beliefs and the necessity for its extension are clarified by referring to the method of knowledge organization and maintenance called ATMS (assumption-based truth maintenance system) introduced by De Kleer, which is an extension of Doyle's TMS. Second, Rescher's plausibility index is briefly reviewed. Third, the index is compared with the probability theoretic index of beliefs by referring to a system of logic called preference logic that reflects essential features of the indices. The comparison clarifies the monotonic, inflexible nature of Rescher's index and the need to extend it in order to cope with the nonmonotonic nature of truth maintenance in dynamically changing situations. An extension of Rescher's plausibility index is introduced, and its properties are examined. Finally, an application of the indexing method to ATMS and to default reasoning is presented, showing that the proposed indexing method can be effectively utilized in truth maintenance of beliefs in complex and dynamically changing situations.