Proving Customary Norms: Ontology and Evidence in International Law

This article addresses the epistemological and conceptual challenges involved in proving customary norms in international law. It argues that confusion surrounding usus and opinio jurisstems from a failure to distinguish between the ontology of customary norms and the determina-tion of their existence. Drawing on legal epistemology, social norm theory, and analytical legal phi-losophy, the article conceptualizes customary norms as products of socially constituted structures grounded in shared normative expectations. Proving opinio juris does not require showing that States regard their conduct as legally mandatory, but rather that second-order normative expecta-tions are manifested through patterns of practice. The article defends a dual inferential model — inductive reasoning to establish usus, complemented by abductive reasoning to infer opinio juris— and proposes a narrative coherence test as a qualitative supplement to the preponderance standard.