A Logical Framework for Default Reasoning

This paper presents a simple logical framework for default reasoning. The semantics is normal first order model theory; instead of changing the logic, the way in which the logic is used is changed. Rather than expecting reasoning to be just deduction (in any logic) from our knowledge, we examine the consequences of viewing reasoning as a very simple case of theory formation. By treating defaults as predefined possible hypotheses we show how this idea subsumes the intuition behind Reiter’s default logic. Solutions to multiple extension problems are discussed. A prototype implementation, called Theorist, executes all of the examples given.