At the beginning of his or her career, every international lawyer has to grapple with the concept of customary international law, with the idea that there are informal, unwritten rules which are binding upon States. This is because there remain important areas of international law, such as the laws of State responsibility and State immunity, where generally applicable treaties do not exist. And despite the lack of an explicit, general consent to rules in these areas, no international lawyer doubts that there is a body of law which applies to them.

I stumbled into the quagmire of customary international law very early in my legal career, in the autumn of 1989. It was during the second year of my law studies when, as a member of McGill University's team in the Jessup International Law Moot Court Competition, I was assigned to write those sections of our memorials that concerned customary international law. Having written what I thought was a thorough analysis of ‘opinio juris’ (i.e., subjective belief in legality) and State practice concerning the issue of maritime pollution in the Antarctic, I was struck by how diffcult it was to explain this ‘law’ to my teammates. They, quite rightly, were concerned about how to present our arguments in a convincing manner, and theoretical discussions of subjective belief seemed far too amorphous to take before judges.