Unpuzzling customary international law (CIL) : the invention of customary law of nations from Francisco de Vitoria to Emer de Vattel

Abstract

This thesis seeks to trace an intellectual history of the concept of customary international law (CIL) within the natural law and ius gentium tradition. Across a timespan of two centuries, in the present work I will make the claim that a strong, foundational relationship exists between the normative content of natural law and the emergence of customary law of nations as a distinctive concept of the international legal discourse. The work is divided in two parts. The first deals with the emergence of the concept of customary law of nations in the early modern 16th century legal tradition, by juxtaposing and contrasting two different natural law doctrines, the theological one of the School of Salamanca (through the eyes of Francisco de Vitoria and Francisco Suárez) and the rhetorical theory of ius gentium by Alberico Gentili. The second part takes into account the modern legal tradition from Hugo Grotius, via Samuel Pufendorf and Christian Wolff, to Emer de Vattel, by showing the relationship between custom and the systematization of natural law into a body of rational law which constitutes a leitmotif of the 17th-18th century. The aim of this work is to assess the argumentative strategies that led to the formation of the concept of customary international law. In other words, the overarching thesis of this project is that the natural law and ius gentium tradition have provided normative content to CIL in ways that are still recognizable today. An intellectual-historical analysis is useful to qualify such content, to show the conceptual development of CIL over time, and ultimately, to answer the question of why CIL is so important to the Western legal tradition of international law.