Resolving conflicts between default rules is a major subtask in performing default reasoning. A declarative way of controlling the resolution of conflicts is to assign priorities to default rules, and to prevent conflict resolution in ways that violate the priorities. This work extends Reiter's default logic with a priority mechanism that is based on lexicographic comparison. Given a default theory and a partial ordering on the defaults, the preferred extensions are the lexicographically best extensions. We discuss alternative ways of using lexicographic comparison, and investigate their properties and relations between them. The applicability of the priority mechanism to inheritance reasoning is investigated by presenting two translations from inheritance networks to prioritized default theories, and relating them to inheritance theories presented earlier by Gelfond and Przymusinska and by Brewka.

An earlier version of the paper in PDF. Elsevier Science B. V. policy forbids the electronic distribution of an up-to-date version of the article by me. The version distributed here differs from the published version: it contains some typos and small errors that are corrected in the one published in Artificial Intelligence Journal (it also looks less professional.)