Practical Reasons and interpretation of Customary International Law

In Panos Merkouris, Jörg Kammerhofer & Noora Arjärvi (eds.), The Theory and Philosophy of Customary International Law and its Interpretation. Cambridge, UK: (forthcoming)
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When we say that we interpret customary international law, what is this thing that we actually interpret? Depending on how we answer this question, our view on interpretative methodology will change. It seems that the most promising approach is to say that interpretation of customary international law is an interpretation of certain legal practices. However, here we also encounter some problems. The dominant doctrine of customary international law requiring state practice and opinio juris assumes that only by adding a psychological element to a regularity of behaviour can we speak of customary international law. That is, opinio juris performs a function of generating both legality and normativity of practices. The article challenges this view by drawing on jurisprudential ideas that define practices as inherently normative. State practices are normative regardless of opinio juris, otherwise they are not practices at all. This view allows to differentiate between two instances of interpretation of customary international law. One is focused on clarification of the normative content of state practices, another is focusing on identification of these practices as legal ones. Interpretation as clarification involves assessing structures of practical reasoning inherent in given practices. Drawing on Joseph Raz's theory of normativity, the article suggests that interpretation of customary rules implies establishing connections between first- and second-order reasons that form the practice and give it meaning. This entails that interpretation of customary international law focuses on dynamics of reasons, their inclusion and exclusion within an existing normative framework. That is, evolutive interpretation of customary international law is not only possible, it is inevitable. Interpretation as identification does not focus on the normative content of practices, but rather involves showing that existing second-order reasons meet a threshold of legal validity. This threshold may take shape of opinio juris, but may as well be treated functionally, when legality of a practice is linked to its relevance in a broader set of legal practices.
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