Customary International Law

Methodology is probably not the strong point of the International Court of Justice (ICJ) or, indeed, of international law in general. Unlike its approach to methods of treaty interpretation, the ICJ has hardly ever stated its methodology for determining the existence, content and scope of the rules of customary international law that it applies. There are only isolated references in the ICJ’s jurisprudence to the inductive and deductive method of law determination. In the Gulf of Maine case, a Chamber of the Court stated that ‘customary international law […] comprises a set of customary rules whose presence in the opinio juris of States can be tested by induction based on the analysis of a sufficiently extensive and convincing practice and not by deduction from preconceived ideas’ ([1984] ICJ Rep 246 [111]). The use of the words ‘can be’, rather than ‘is’, implies that customary international law rules can also be discovered deductively. That deduction is part of the ICJ’s methodological arsenal is demonstrated by the fact that in the North Sea Continental Shelf cases five…