This article tackles the issue of whether and how Hugo Grotius conceives of custom as a formal source of the law of nations. The main claim of it is that not only custom plays a fundamental role in Grotius’s thought, but that his reflections mark a fundamental turning point for the history of customary international law. A crucial role in this process of re-conceptualization is played by Grotius’s reading of Dio Chrysostom, whose oration On custom provides him with an integrated account of custom as a ‘normative practice’ based on rhetorical judgment (as opposed to the Scholastic interpretation of custom as reiteration of voluntary acts). Consequently, I argue that Dio Chrysostom’s text helps Grotius to transpose the question of the normative legitimacy of custom from a moral to an interpretative level. To conclude, I will show that Grotius adopts two different rhetorical strategies to prove the existence of customary norms of ius gentium.
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