The Romans described their legal system as consisting of two ingredients. "All nations," says the Institutional Treatise published under the authority of the Emperor Justinian, "who are ruled by laws and customs, are governed partly by their own particular laws, and partly by those laws which are common to all mankind. The law which a people enacts is called the Civil Law of that people, but that which natural reason appoints for all mankind is called the Law of Nations, because all nations use it." The part of the law "which natural reason appoints for all mankind" was the element which the Edict of the Prætor was supposed to have worked into Roman jurisprudence. Elsewhere it is styled more simply Jus Naturale, or the Law of Nature; and its ordinances are said to be dictated by Natural Equity (_naturalis æquitas_) as well as by natural reason. I shall attempt to discover the origin of these famous phrases, Law of Nations, Law of Nature, Equity, and to determine how the conceptions which they indicate are related to one another.
The most superficial student of Roman history must be struck by the extraordinary degree in which the fortunes of the republic were affected by the presence of foreigners, under different names, on her soil. The causes of this immigration are discernible enough at a later period, for we can readily understand why men of all races should flock to the mistress of the world; but the same phenomenon of a large population of foreigners and denizens meets us in the very earliest records of the Roman State. No doubt, the instability of society in ancient Italy, composed as it was in great measure of robber tribes, gave men considerable inducement to locate themselves in the territory of any community strong enough to protect itself and them from external attack, even though protection should be purchased at the cost of heavy taxation, political disfranchisement, and much social humiliation. It is probable, however, that this explanation is imperfect, and that it could only be completed by taking into account those active commercial relations which, though they are little reflected in the military traditions of the republic, Rome appears certainly to have had with Carthage and with the interior of Italy in pre-historic times. Whatever were the circumstances to which it was attributable, the foreign element in the commonwealth determined the whole course of its history, which, at all its stages, is little more than a narrative of conflicts between a stubborn nationality and an alien population. Nothing like this has been seen in modern times; on the one hand, because modern European communities have seldom or never received any accession of foreign immigrants which was large enough to make itself felt by the bulk of the native citizens, and on the other, because modern states, being held together by allegiance to a king or political superior, absorb considerable bodies of immigrant settlers with a quickness unknown to the ancient world, where the original citizens of a commonwealth always believed themselves to be united by kinship in blood, and resented a claim to equality of privilege as a usurpation of their birthright. In the early Roman republic the principle of the absolute exclusion of foreigners pervaded the Civil Law no less than the Constitution. The alien or denizen could have no share in any institution supposed to be coeval with the State. He could not have the benefit of Quiritarian law. He could not be a party to the _nexum_ which was at once the conveyance and the contract of the primitive Romans. He could not sue by the Sacramental Action, a mode of litigation of which the origin mounts up to the very infancy of civilisation. Still, neither the interest nor the security of Rome permitted him to be quite outlawed. All ancient communities ran the risk of being overthrown by a very slight disturbance of equilibrium, and the mere instinct of self-preservation would force the Romans to devise some method of adjusting the rights and duties of foreigners, who might otherwise--and this was a danger of real importance in the ancient world--have decided their controversies by armed strife. Moreover, at no period of Roman history was foreign trade entirely neglected. It was therefore probably half as a measure of police and half in furtherance of commerce that jurisdiction was first assumed in disputes to which the parties were either foreigners or a native and a foreigner. The assumption of such a jurisdiction brought with it the immediate necessity of discovering some principles on which the questions to be adjudicated upon could be settled, and the principles applied to this object by the Roman lawyers were eminently characteristic of the time.