Legal rules are either canonical or malleable. A canonical rule is fixed: it may not be expressed in different ways, may not evolve, and may not be subject to exceptions. Statutes are the paradigm form of canonical legal rules. In contrast, a malleable rule can be expressed in different ways, can evolve, and can be made subject to exceptions. Common law rules are the paradigm form of malleable rules. Legal rules have a core and a penumbra. A common law rule can be articulated through more than one expression of the core of the rule, through evolution of the meaning of the core, or through modification of the rule’s penumbra. In each case the core remains, albeit in somewhat different forms. As Benjamin Cardozo said, “The rules and principles of case law have never been treated as final truths but as working hypotheses, continually retested in those great laboratories of law, the courts of justice.”