An Approach to Context-Based Default Reasoning

In this paper, we elaborate the idea that contexts provide an important and meaningful notion in default reasoning. We demonstrate this by looking at Reiter's default logic that has been the prime candidate for formalizing consistency-based default reasoning ever since its introduction in 1980. This results in a new context-based approach to default logic, called contextual default logic. The approach extends the notion of a default rule and supplies each default extension with a context. In particular, contextual default logic provides a unified framework for default logics. That is, it allows for embedding existing variants of default logic along with more traditional approaches like the closed world assumption. Since this is accomplished in a homogeneous way, we gain additional expressiveness by combining the diverse approaches. A key advantage of contextual default logic is that it provides a syntactical instrument for comparing existing default logics in a unified setting. In particular, the approach reveals that existing default logics mainly differ in the way they deal with an explicit or implicit underlying context.