ABSTRACT
This chapter examines the theoretical construct of the idea of custom and its application in the formation of rules of customary law in the international legal system. It confirms the legitimacy deficit in customary international law, and informs attempts to reformulate custom of the issues. The chapter shows that both treaty and usage origins of custom premise the emergence of a norm of customary international law on the consummation of State practice (SP) and a belief that that particular practice was obligatory – also known as opinio juris sive necissitatis (OJ). More importantly, it shows that far from making certain the distinction between legal customs and mere usage this definition of custom does not fully account for the process by which norms of customary international law are created. Brierly correctly observes that it was positivism that helped draw the positivist distinction between law and ethics.
