The "Jus Gentium"
(Primary Reference: Will Durant, The Story of Civilization: Part III, Caesar and Christ, A History of Roman Civilization and of Christianity from their beginnings to A.D. 325, Simon and Schuster, NY c1944.)
Non-citizens in Rome were not entitled to the privileges and protections afforded to the citizen under Roman Law. The institution of citizenship placed the individual on a status coequal with the state. The alien or "denizen" was placed on a different level. The ius Civile, or Roman Civil Law, absolutely excluded non-citizens from the "nexum" or mode of contract and conveyance, rendering the foreigner incapable of dominium. He could not have the benefit of Quiritarian law. He could not sue by the Sacramental Action, (a mode of litigation.) Necessity in adjudicating conflicts involving foreigners required the creation of a separate body of law.
The inevitable need to assume jurisdiction over denizen traders brought with it the immediate necessity of discovering some principles upon which questions could be settled. Rome refused to apply the law of the particular Italian (non-Roman) State from native to the foreigner. Instead, it appointed a praetor peregrimus to settle matters pertaining to the first Italian foreigners in Rome. Power was given to him to make a union between Roman and foreign laws. The praetor peregrimus then selected rules of law common to both Rome and to the different Italian communities in which the immigrants were born. The annual edicts of this praetor and the later provincial governors of the Roman Empire created the "ius (Jus) Gentium," or "Law of Nations" (Law common to all Nations.)
The Jus Gentium was comprised of common ingredients in both the customs of the old (non-Roman) Italian tribes and the new (non-Italian) provinces that were compatible with Roman concepts of law. Whenever a particular usage was seen to be practiced by a large number of separate cultures in common with Roman Law, it was set down as part of the Law. The Jus Gentium was designed to govern the peoples of Italy and the provinces, without giving them Roman citizenship and the other rights of the ius (Jus) civile.
For instance, although the conveyance of property was ceremonialized in very different forms in the different Italian commonwealths surrounding Rome, the actual transfer ("tradition", or delivery,) of the article intended to be conveyed was a part of the ceremony in each place. "Tradition," therefore, was set down as an institution in the Juris Gentium, or rule of the Law common to all Nations.
The Stoics created the fiction that identified the Law of Nations with the "Law of Nature." The Law of Nature, or Natural Law, was considered a moral code implanted in man by "natural reason." The Stoics believed that this sense of logic and order in all things spontaneously developed in society and, when consciously expressed, was "Natural Law." Modern international law is based on the assumption that this Natural Law governs the inter-relations of independent nations. (The Jus Gentium should not be confused with the international law or Jus Feciale - the law of negotiation and diplomacy.)