It is often thought that the judicial recognition of customary international law depends on jurisprudential assumptions about the nature of legal norms, law, and legal validity. This is a mistake. The limits on a judicial reliance on customary international law are constitutional or evidentiary, not jurisprudential. Although Erie Railroad Co. v. Tompkins fairly can be read to require domestic authorization for customary international law to have domestic legal effect, the case and its reasoning do not rely on commitments to a theory of law. Acceptance of the claims of legal positivism is neither necessary nor sufficient for Erie’s result, nor for its application to customary international law. In fact, reliance on positivism has an unwelcome consequence for the legally binding character of customary international law even on states. Finally, the same conception of law or legal validity can ground different views about the relation between international and domestic law. Positions on the priority of customary international law are determined by views about that relation, not by views on the source of its authority. Taken together, these considerations suggest that jurisprudence isn’t needed to answer the questions that courts and legal authorities ask about customary international law's content, the legal obligations it creates, and its domestic legal effect.

Steven D. Walt, Why Jurisprudence Doesn’t Matter for Customary International Law, 54 William & Mary Law Review, 1023–1055 (2013).
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