A Theory of Customary International Law

Abstract

A satisfactory theory of customary international law should be able to explain the role of international agents' attitudes towards their practices (their opinio juris) in the formation of customary law; to arbitrate disagreements not just about whether a certain act `counts' towards the formation of a customary rule, but also about how customary practices should be interpreted; to explain the changes in the ways international lawyers argue about customary law; and to show customary international rules as a response to pressing moral and political concerns. I claim that extant theories of customary international law fall decidedly short of the above desiderata. First, their treatment of the two elements of customary law is both unhelpful and misleading. Second, they are unable to resolve some of the commonest and politically pressing disagreements about the legal character and the proper interpretation of customary practices. Third, they misunderstand the nature of the change in argumentative practice about customary international law. And fourth, they make insufficient space for the role of values in the formation and the interpretation of customary international practices. My paper backs up these assertions by proposing a constructive account of customary international law and is much better placed to meet those desiderata. It argues that questions about the formation and interpretation of customary international practices require us to articulate the purpose or value that the practices in question are best understood as serving and to reconstruct our conception of the particular rules of the practice in that light.