Part I Prolegomena Chapter I The Evolution towards a New Jus Gentium: The International Law for Humankind I. The Historical Emergence of Jus Gentium Jus gentium (of Roman law) had its origins in private law,1 having subsequently entered also the realm of the conceptual universe of the law of nations.2 It origi- nally consisted of common principles governing legal relations in general. Grad- ually the concept of jus gentium was enlarged, so as to encompass what came to be known as the general principles of law.3 In the XVIth century, Francisco de Vitoria conceived the new jus gentium of his days as governing the relations of all peoples (including the indians of the new world) and individuals, in conditions of independence and juridical equality, pursuant to a universalist outlook (totus orbis). In a world marked by diversifi cation (of peoples and cultures) and plural- ism (of ideas and cosmovisions), the new jus gentium secured the unity of the societas gentium.4 It could not possibly be derived from the “will” of its subjects, but rather based on a lex praeceptiva, apprehended by human reason. In Vitoria’s view, jus gentium was applicable to all peoples and human beings (even without the consent of its addressees); societas gentium was the expression of the funda- mental unity of humankind, wherefrom jus gentium ensued, and provided the juridical basis (deriving from a lex praeceptiva of natural law) for the totus orbis, 1 On the concept of jus gentium in classic Roman law, cf., e.g., Gabrio Lombardi, Ricerche in Tema di “Ius Gentium”, Milano, Giuff rè, 1946, pp. 3-272; Gabrio Lom- bardi, Sul Concetto di “Ius Gentium”, Roma, Istituto di Diritto Romano, 1947, pp. 3-390. 2 Having originally applied among citizens, and in their relations with foreigners, jus gentium was subsequently - with Cicero - identifi ed as the law common to all peoples, ultimately as the law common to all mankind; H.F. Jolowicz, Historical In- troduction to the Study of Roman Law, 2nd. ed., Cambridge, University Press, 1967 [reed.], pp. 102-105; and cf. also, e.g., W. Kunkel, Historia del Derecho Romano, 9th. ed., Barcelona, Ed. Ariel, 1999, pp. 85-87. 3 H. Mosler, “Th e International Society as a Legal Community”, 140 Recueil des Cours de l’Académie de Droit International de La Haye [RCADI] (1974) p. 137. 4 It was no longer a jus divinum, nor the jus civile (no longer the Pope, nor the Em- peror, as the ultimate authority), but rather the jus gentium, defi ned by Francisco de Vitoria as quod naturalis ratio inter omnes gentes constituit, vocatur jus gentium..