"This essay examines the ambition to taxonomize law and the different methods a legal taxonomer might employ. Two possibilities predominate. The first is a reason-based taxonomy that classifies legal rules and decisions according to “legal principles” thought to justify them. Reason-based taxonomy of this type offers courts a set of high-level decisional rules, drawn from legal data, for use in deciding new cases and evaluating precedents. The second possibility is a formal taxonomy that classifies legal materials according to rules of order and clarity. Formal taxonomy serves less ambitious objectives, such as facilitating legal analysis and communication. It does not provide decisional standards for courts. I conclude that reason-based taxonomy, classifying law according to legal principles, is a misguided enterprise. It may be more satisfying to the taxonomer, but it will not improve the process or outcomes of judicial decisionmaking."