Manifest Intent and the Generation by Treaty of Customary Rules of International Law

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The World Court’s recent decision in the North Sea Continental Shelf Cases is a major contribution to that branch of the theory of customary international law dealing with norm-creation by means of a treaty. The Court articulated a new methodology for determining which provisions in treaties can form the basis of universally binding customary law. As the spreading network of international conventions becomes more fine-meshed, the substantive rules of international customary law may be expected to conform more and more closely to the provisions in these conventions. The World Court has implicitly recognized this process in many prior opinions, but it was not until the Continental Shelf decision that the link between treaty and custom was focused upon with precision. I shall argue in this essay that the Court used a method which might be called the rule of manifest intent, that this method differs from a more traditional approach found in the writings of publicists, and that this new method accords well with the growing need to objectify and place upon a scientific basis the methodology by which one may determine what in fact are the rules of customary law.