The notion of ius gentium has been mostly studied starting from the definitions of jurists, influenced by Greek philosophy. This has led the modern interpreters to believe in a fairly recent origin of the notion, and this belief has been strengthened by the idea that the institutes of ius gentium were born in praetorian law, being later incorporated into the ius civile. However, the hypothesis of a praetorian origin of the institutes is based on the projection, on antiquity, of a positivistic vision of law. An analysis that starts from the actual working of the institutes shows that what is read as ‘reception’ or even as ‘juridification’ of the commercial practice of the Mediterranean is nothing other than the inclusion of such practice in already existing schemes of the ius civile, by means of a sort of interpretatio Romana.