Logic Seminar: Session 9

Rule-based legal reasoning, as exemplified by the reasoning needed to apply complex statutes such as the Internal Revenue Code, is usefully conceived of as defeasible reasoning, and thus usefully represented using a nonmonotonic logic such as default logic. Extant versions of default logic, however, such as the order-of-application approach due to Brewka and Eiter, and the fixed-point approach due to Horty, are both too complex and underspecified to represent rule-based legal reasoning effectively. The set of legal rules from which one reasons, for example, is finite, and additional restrictions are placed on what may trigger the application of rules. This talk proposes a version of default logic, what Lawsky calls RBR-logic, that accurately and efficiently represents rule-based legal reasoning.