10 - Three ways of writing a treatise on public international law: textbooks and the nature of customary international law

Summary

International law is a system of customary law. And, like all writers of textbooks on customary law, scholars of international law find themselves in a curious position: it is philosophically impossible to treat customary law as a system of clear, settled rules since there is no way of settling the correct text or formulation of those rules. As Brian Simpson writes in an essay on English common law,

we all know that no two legal treatises state the law in the same terms, there being a law of torts according to Street, and Heuston, and Jolowicz and James and the contributors to Clerk and Lindsell, and we buy them all because they are different. And what is true of the academics is true perhaps even more dramatically of the judges, who are forever disagreeing, often at inordinate length … As a system of legal thought the common law then is inherently vague; it is a feature of the system that uniquely authoritative statements of the rules which, so the positivists tell us, comprise the common law, cannot be made.

What is true of English law is at least as true of public international law, on which not only academics and individual judges but also tribunals and courts disagree, similarly often at inordinate length.

This essay argues that one of the most fundamental differences between various treatises on international law stems from diverging assumptions about what renders a statement of a rule of international law correct or authoritative.