The Limits of Deduction in the Identification of Customary International Law

Much scholarship on customary international law has examined the merits of induction, deduction, and assertion as approaches to custom identification. Save for where international tribunals identify custom by assertion, writers have viewed custom identification that does not rely on evidence of State practice and opinio juris as an example of deductive reasoning. However, writers have stated that, at best, deduction is reasoning from the general to the particular. This article draws on legal philosophy to define the contours of deductive reasoning and argues that pure deduction, namely deduction not combined with other forms of reasoning, is an unsound approach to custom identification. This argument is tested by reference to cases of custom identification by the International Court of Justice, categorised according to three types of deduction: normative, functional, and analogical. This article also explores the authority and utility of custom identification by pure deduction and its impact on content determination.