Statistical Foundations for Default Reasoning

Abstract

We describe a new approach to default reasoning, based on a principle of indifference among possible worlds. We interpret default rules as extreme statistical statements, thus obtaining a knowledge base KB comprised of statistical and first-order statements. We then assign equal probability to all worlds consistent withKB in order to assign a degree of belief to a statement'. The degree of belief can be used to decide whether to defeasibly conclude'. Various natural patterns of reasoning, such as a preference for more specific defaults, indifference to irrelevant information, and the ability tocombine independent pieces of evidence, turn out to follow naturally from this technique. Furthermore, our approach is not restricted to default reasoning; it supports a spectrum of reasoning, from quantitative to qualitative. It is also related to other systems for default reasoning. In particular, we show that the work of [Goldszmidt et al., 1990], which applies maximum entropy ideas to-semantics, can be embedded in our framework.