A Theory of Interpretation for Customary International Law

Customary international law (CIL), alongside treaties and general principles, is one of the three primary sources of international law. Historically, rules of CIL are some of the oldest rules of international law, forming the foundations of the system. While scholarship has dealt extensively with questions surrounding the creation and identification of customary rules, less attention has been paid to their interpretation. At the same time, a survey of the practice of domestic and international courts shows that courts frequently engage in the interpretation of customary rules separate from their identification. The existence of this practice calls for a deeper engagement with the question of CIL interpretation.The question of CIL interpretation is a question about how we determine the scope and content of customary rules. The main claim of this thesis is that customary rules can be subject to interpretation, and that interpretation is a specific and separate operation in the continued existence of customary rules, different from their identification. Furthermore, interpretation performs two crucial functions in the continuous existence of customary rules, and accounting for interpretation is both theoretically relevant and practically necessary. In particular, interpretation performs a concretizing function – whereby the scope and content of general customary rules is delineated and made specific, and an evolutive function – whereby older customary rules are updated in light of new factual or legal developments. The two functions that interpretation performs are not mutually exclusive.Accounting for interpretation enables us to better understand the way customary rules function, and the way they are applied in the practice of international law. This thesis offers a theory of interpretation for customary international law that engages with both the doctrinal and the practical aspects of this practice, and provides a systematic account of the role of interpretation in the continued existence of customary rules