Conceptions of Custom in International Law

Abstract

International legal theory has long relied upon the institution of custom to explain one of the purported 'sources' of international law. That reliance, however, has been largely devoid of philosophical attention to the institution of custom itself, instead seeking to formalise the method through which 'non-legal' custom can be transformed into 'law'. This paper advances the proposition that such an approach adopts an unnecessarily narrow view of the concept of custom, and results in the neglect of alternative conceptions of custom and their potential manifestations in international law. This article examines how custom has been conceived in philosophical analysis in both domestic jurisprudence and in international legal theory. The results reveal not one but three conceptions of custom, concerned respectively with treating custom as a source of law; as a mode of order in its own right; and as a foundation of interaction from which law is generated. These conceptions are then evaluated for their usefulness in explaining custom in the international system, concluding that that international legal theory cannot rely upon only one of the conceptions if it is to explain and utilise custom as a legal concept. Instead, this paper offers a reconception of 'compound custom' which links the foundational social element of custom with its de facto operation and the more formal processes that generate obligations. Compound custom offers an alternative basis from which international legal theory may seek to build new theories and doctrines to explain what is truly customary about customary international law.