In most modern legal systems, legal rules are widely understood as defeasible, in the sense that the prescriptions of the legal rule may legitimately be overridden or otherwise cancelled when those prescriptions appear to generate substantially erroneous results. As a result, it is common for commentators, including H.L.A. Hart and Richard Posner, to treat the defeasibility of legal rules as an essential feature of rules or an essential feature of law. But defeasibility is in important ways in tension with the goals of the rule of law, and so although there may be good reasons for a legal system to treat some or all of its rules as defeasible, there are also good reasons for refusing to do so. And so long as this is the case, it is a mistake to treat legal defeasibility as essential to law, for doing so both makes a mistake about the nature of law and deprives institutional designers of the advantages that may sometimes come from non-defeasible rules.