The legal literature has also had little to say on this subject. It is not only the ICJ itself that has largely remained silent on its methodology for the determination of customary international law. That is the deduction of the necessity of this principle from the fundamental concept of the continental shelf.’ 6 In the Court’s more recent jurisprudence, the Arrest Warrant case is widely seen as an example of deductive reasoning, 7 while the Jurisdictional Immunities of the State case is regarded as a prime example of the Court using the inductive method. 5 For example, Judge Lachs stated that in ‘the event that the customary law character of the principle of equidistance cannot be proved, there exists another reason which seems more cogent for recognizing this character. That deduction is part of the Court’s methodological arsenal is demonstrated by the fact that in the North Sea Continental Shelf cases five judges used the deductive method in their separate or dissenting opinions.
4 The use of the words ‘can be’, rather than ‘is’, implies that customary international law rules can also be discovered deductively. 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’. 3 In the Gulf of Maine case, a Chamber of the Court stated that ‘customary international law. 2 There are only isolated references in the ICJ’s jurisprudence to the inductive and deductive method of law determination. 1 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. Methodology is probably not the strong point of the International Court of Justice (ICJ) or, indeed, of international law in general. 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. 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. It starts by defining the terms ‘induction’ and ‘deduction’ and examining their use by the Court. 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. 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.
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. There are only isolated references in the Court’s jurisprudence to the inductive and deductive method of law determination. 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. Methodology is probably not the strong point of the International Court of Justice or, indeed, of international law in general.