A Logic for Statutes

Case-based reasoning is, without question, a puzzle. When students are taught to “think like lawyers” in their first year of law school, they are taught case-based common-law reasoning. Books on legal reasoning are devoted almost entirely to the topic. How do courts reason from one case to the next?

Is case-based reasoning from analogy? How should case-based reasoning be modeled? How can it be justified? In contrast, rule-based legal reasoning (as exemplified in much statutory reasoning) is taken as simple in legal scholarship.

Statutory interpretation—how to determine the meaning of words in a statute, the relevance of the lawmakers’ intent, and so forth—is much discussed, but there is little treatment of the structure of statutory reasoning once the meaning of the words is established. Once the meaning of terms is established, statutory reasoning is considered, roughly speaking, to be deductive reasoning. This Essay examines the structure of statutory reasoning after ambiguities are resolved and the meaning of the statute’s terms established. It argues that standard formal logic is not the best approach for modeling statutory rule-based reasoning.

Rather, the Essay argues, using the Internal Revenue Code and accompanying regulations, judicial decisions, and rulings as its primary example, that at least some statutory reasoning is best characterized as defeasible reasoning—reasoning that may result in conclusions that can be defeated by subsequent information—and is best modeled using default logic. The Essay then addresses the practical and theoretical benefits of this alternative understanding of rule-based legal reasoning.