 If law cannot ultimately be grounded in force, or in a presupposed norm, on what does its authority rest? The most influential solution is perhaps H.L.A. Hart’s. His solution resembles Kelsen’s in its emphasis on the normative foundations of legal systems, but Hart rejects Kelsen’s transcendentalist, Kantian view of authority in favor of an empirical, Weberian one. For Hart, the authority of law is social. The ultimate criterion of validity in a legal system is neither a legal norm nor a presupposed norm, but a social rule that exists only because it is actually practiced, that is, used to guide conduct. Law ultimately rests on custom: customs about who shall have the authority to decide disputes, what they shall treat as binding reasons for decision, i.e., as sources of law, and how laws may be changed. Of these three “secondary rules”, as Hart calls them, the source-determining rule of recognition is most important, for it specifies the ultimate criteria of validity in the legal system. It exists only because it is practiced by officials, and it is not only that the recognition rule best explains their practice, it is the rule to which they actually appeal in arguments about what standards they are bound to apply. Hart’s account is therefore in one sense conventionalist (see Marmor 1998 and Coleman 2001): ultimate legal rules are social norms, although they are neither the product of express agreement nor conventions in the Schelling-Lewis sense (see Green 1999). Thus for Hart too the legal system is rule-based all the way down, but at its root is a social norm that has the kind of normative force that customs have. It is a regularity of behavior regarding which officials take “the internal point of view:” they use it as a standard for guiding and evaluating their own and others’ behavior, and this use is displayed in their conduct and speech, including the resort to various forms of social pressure to support the rule and the ready application of normative terms such as “duty” and “obligation” when invoking it.