Nonmonotonicity and the Scope of Reasoning: Preliminary Report

Abstract

Existing formalisms for default reasoning capture some aspects of the nonmonotonicity of human commonsense reasoning. However, Perlis has shown that one of these formalisms, circumscription, is subject to certain counterintuitive limitations. Kraus and Perlis suggested a partial solution, but significant problems remain. In this paper, we observe that the unfortunate limitations of circumscription are even broader than Perlis originally pointed out. Moreover, these problems are not confined to circumscription; they appear to be endemic in current nonmonotonic reasoning formalisms. We develop a much more general solution than that of Kraus and Perlis, involving restricting the scope of nonmonotonic reasoning, and show that it remedies these problems in a variety of formalisms. Introduction The search for theories of nonmonotonic reasoning--- theories of how to reach reasonable conclusions that are not strictly entailed by what is known, and hence are subject to retraction---has yielde...