Chapter One. Customary International Law, Theoretical Conceptions And Evidence Of Its Formation

This chapter explores the notion of custom and its function as a source of international law. It refers to the different theories on the formation of custom and to the underlying idea of the nature of general international law. Article 38 (1) (b) of the ICJ Statute sets out two elements which underlie a norm of customary international law: opinio juris and state practice. Although there is an immense amount of controversial literature produced on the formation of customary norms, the only thing universally agreed upon is the category of international law created through custom. It is essential to view the different theories on custom formation in the broader context of their understanding of the nature of international law. The chapter provides a discussion of the theory of customary international law divided according to the major philosophical schools currently existing in international legal doctrine.Keywords: custom formation; customary international law; ICJ Statute; opinio juris; state practice