Determining Customary International Law: The ICJ's Methodology between Induction, Deduction and Assertion


Methodology is probably not the strong point of the International Court of Justice or, indeed, of international law in general. Unlike its approach to methods of treaty interpretation, the Court 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 Court’s jurisprudence to the inductive and deductive method of law determination. It is not only the Court itself that has largely remained silent on its methodology for the determination of customary international law, but the legal literature also has had little to say on this subject. In view of the fact that determining the law has also always meant developing, and ultimately creating, the law it is surprising that the question of the Court’s methodology has attracted such little interest. This article aims to refocus attention on the methodology used by the Court when determining the rules of customary international law that it applies, and it highlights the role played by methodology in the development of customary international law. It starts by defining the terms ‘induction’ and ‘deduction’ and examining their use by the Court. It then explores the situations in which the Court uses inductive and deductive reasoning, the different forms and functions of deduction and the relationship between the two methods. The article challenges the various theories distinguishing between inductive and deductive custom and demonstrates that the main method employed by the Court is neither induction nor deduction but, rather, assertion.

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