Default Logic

Abstract

Default logic is an important method of knowledge representation and reasoning, because it supports reasoning with incomplete information, and because defaults can be found naturally in many application domains, such as diagnostic problems, information retrieval, legal reasoning, regulations, specifications of systems and software, etc. Default logic can be used either to model reasoning with incomplete information that was the original motivation or as formalism that enables compact representation of information. This chapter discusses the basic concepts and ideas of default logic, and the presentation was based on the operational interpretations, rather than on fixpoints. The operational interpretations allow learners to apply concepts to concrete problems in a straightforward way. This is an important point because the difficulty of understanding default logic should not be underestimated. In some cases, standard default logic is insufficient to resolve conflicts among defaults. Preferences provide a declarative way to solve this problem and thus many approaches to preference handling in default logic are proposed.