D'Aspremont, J., The Discourse on Customary International Law, Oxford, Oxford University Press, 2021

Along with treaties, custom is one of the sources of international law. It is known to consist of two elements: state practice and opinio juris. While many studies have looked at traditional questions of how to identify customary law, this book takes a new and original approach. It looks instead at the structure of thought that lies beneath the arguments about customary international law. By examining these structures, the book uncovers surprising conclusions, and demonstrates what the author describes as the 'discursive splendour' of customary international law.

The book guides the reader through an analysis of eight distinct performances at work in the discourse on customary international law. One of its key claims is that customary international law is not the surviving trace of an ancient law-making mechanism that used to be found in traditional societies. Indeed, as is shown throughout, customary international law is anything but ancient, and there is hardly any doctrine of international law that contains so many of the features of modern thinking.