Legal contractions: A logical analysis

Abstract

This paper systematically investigates how to model legal contraction in an expressive variant of Defeasible Deontic Logic. We argue that legal contraction is an umbrella concept that includes operations which are conceptually and technically different: removing rules, adding exceptions, and modifying rule priorities. The peculiarities of deleting legal conclusions show that an extension of those operations is sometimes needed, which works on the indirect conclusions from which the target effect of the contraction is obtained. The proposed techniques are discussed in the context of a new version for the logic of AGM postulates of contraction.