This article is concerned with the debate about interdisciplinary methods in international law, in particular the turn to International Relations. It finds the historical critique of Martti Koskenniemi grounded in a more methodological issue: the turn toward a redefinition of norm properties impedes on the critical discursive quality of law. Shaping this historical critique into a research question that allows for meaningful engagement, the article discusses Koskenniemi's charges drawing on

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... nt constructivist scholarship. Giving an account of what it means to be 'obliged' to obey the law, this article defends the coherence of Koskenniemi's position and suggests that we should take the critique of the interdisciplinary project between law and International Relations seriously. While it agrees that a significant part of the discourse fails to appreciate the particularities of the law, it suggests that understanding legal obligations requires taking the institutional autonomy of the law into account. Respecting this autonomy, in turn, points to a multi- instead of an interdisciplinary project. The reflexive formalist conception of the law that this article advocates captures the obligating nature of the law, independent of the normative content of particular rules.