A Mystery No Longer?Opinio Jurisand Other Theoretical Controversies Associated with Customary International Law

Offering a current reflection on Raphael Walden's 1977 article, ‘The Subjective Element in the Formation of Customary International Law’, this contribution seeks to illustrate that considerable clarity has been achieved over the decades with regard to several long-standing questions associated with customary international law, not least those surroundingopinio juris. Accumulated practice and constructive scholarship have supplied insights into, and indeed answers to several of the controversies that have bedevilled the theory of this central source of international law. While it may inherently defy exact formulations, and some theoretical questions remain, customary international law is thus today not only as present in the international legal system as it has always been but is also better understood.