Eureka! On Courts' Discretion in 'Ascertaining' Rules of Customary International Law

The ascertainment of rules of customary international law has been typically inserted in the methodological dualism between induction and deduction. By induction, lawyers find existing legal rules on the basis of the empirical material – state practice and opinio juris. By deduction, instead, lawyers find legal rules by deducing them from existing principles and rules of international law. Notably, while induction is portrayed as empirically grounded – therefore objective – deduction is presented as a logical exercise, thus disguising the margin of manoeuvre that interpreters enjoy in ascertaining rules of customary international law. The present contribution contends that the methodological dualism informing the discourse on the ascertainment of rules of customary international law shall be revisited to reflect the argumentative nature of such an ascertainment. The alternative dualism between discovery and justification is thus proposed. This twist is conducive to unveil the range of discretion exercised by courts in the formulation of rules of customary international law, instead of embracing the methodological lens which rather mystifies it. Accordingly, courts operate within an argumentative framework in that they necessarily select and appreciate evidence of practice and opinio juris, which is far from being incontrovertible, let alone fully representative of the majority of states. In other terms, the selection and assessment of practice and opinio juris are but the result of an act of judicial discretion, which is obscurely surfacing every act of legal interpretation. Yet, owing to the authority of courts in a legal order, their formulation of presumably existing rules of customary international law is a necessary endeavour for the materialization of such ‘rules’ and their fruition by the legal practice.