Normal Forms for Defeasible Logic

Defeasible logic is an important logic-programming based nonmonotonic reasoning formalism which has an eecient implementation. It makes use of facts, strict rules, defeasible rules, defeaters, and a superiority relation. Representation results are important because they can help the assimilation of a concept by connning attention to its critical aspects. In this paper we derive some representation results for defeasible logic. In particular we show that the superiority relation does not add to the expressive power of the logic, and can be simulated by other ingredients in a modular way. Also, facts can be simulated by strict rules. Finally we show that we cannot simplify the logic any further in a modular way: Strict rules, defeasible rules, and defeaters form a minimal set of independent ingredients in the logic.