This 'law of the men of all nations' (ius gentium) was of course not an international law, it was a law administered by Roman officers, and it was coloured by Roman conceptions, however much it may have drawn from a comparison of foreign laws with which the Romans were brought into contact. In turn it reacted upon the more narrow law of Roman citizens (ius civile), broadening its conceptions and enabling it to free itself from primitive formalism. It also made easier the task of Roman governors who were called upon to administer the various laws of the different countries which came to form the Roman empire.
The gradual extension of the citizenship (completed at the end of the second century A.D.) to the whole of the inhabitants of the empire made possible, at least in outward appearance, the application of a uniform system of law throughout what was then the civilized world, though beneath an apparent uniformity local traditions and customs survived to the end, at any rate in the east. The 'civil law', as the Roman law in its final form has been called down to the present day, consists of elements of the narrowly Roman and the more universal law inextricably interlaced.
This Roman solution of the problem of the foreign litigant is of much more than merely practical importance. The Stoic philosophy which grew up amid the decay of the old city life, whose adherents spoke of themselves as citizens of the world, had fastened upon the old antithesis of law (or convention) and nature, and formed the conception of a law of nature, which should have a reasonable basis and a validity superior to the arbitrariness of the city law. To this ideal conception the Roman law of the men of all nations gave a body and a reality. Stoicism became the 'established' philosophy of Rome, and Roman lawyers well-nigh identified the 'ius gentium' with the ideal law of nature, describing it as that which natural reason has established among all men. Yet for at least one of the great classical lawyers, whose words have been enshrined in Justinian's legislation, the identification was incomplete. By nature, it was said, all men are free, and mankind has departed from what natural reason requires, in permitting slavery. Thus the law of nature must be sought in something more universal than the practice of mankind. More than fifteen hundred years later in an English court an argument against the recognition of the rights of a slave-owner was successfully founded on the law of nature.
Before the Roman law had been put (at Constantinople) into the final shape in which it is preserved to us, the Roman empire in the west had already been broken up by barbarian invasions. The invaders brought with them their tribal laws and customs, rude, often cruel, narrow rather than simple, for simplicity is the work of civilization. They did not understand, and could not adopt, the law of the world into which they had come. Yet neither could they, if they would, force their laws upon the conquered inhabitants. Among these the old civilization lingered on in a degenerate form, and with it the Roman law. One of the first things that happened was that the conquerors drew up for their Roman subjects short codes of the Roman law as it survived in a debased form, as they drew up statements of their own law for their followers. For a long time each man, according to the community to which he belonged, had a 'personal' law. As late as A.D. 850 we hear that in France it might happen that five men met together and each would have a different law. Of course such a state of things means before very long that there must be at any rate one set of common legal rules which must be applied throughout a territory, namely rules to decide which kind of personal law is to be used when there is a dispute between two persons whose personal law is different.
Gradually the different populations within the same area coalesce, and law from being personal becomes local. But the local area will not be the same for all purposes. The law or custom which determines the rights of the small, often unfree or half-free tenant, whether as between him and his neighbour or as between him and his lord, may extend no further than a very small area, such as in England we call a manor. The law by which great men held their land from a king, though perhaps not uniform throughout the kingdom, will cover a much larger area. The fact that a great man may hold land in far distant places, it may be in different kingdoms, and that men of this class have connexions with different parts of Western Europe will lead to the formation of common notions of feudal law, which make possible even the scientific study of a law of feuds, though no complete uniformity was ever attained.
England was the first western country to attain political unity with a territory substantially the same as at the present day; and the determination of the English kings that in the more important matters justice should be done throughout the land in the king's name, either by his courts at Westminster or by judges sent by him to the counties, secured the formation of an English Common Law which left comparatively little play for local custom, and which at an early time became strong enough to resist attempts to introduce foreign law. As early as the time of Henry III the barons proclaimed with one voice that they would not have the laws of England altered in favour of a rule—the legitimation of bastards by the subsequent marriage of their parents—which in one form or another has been adopted in Western Christendom, and even in the neighbouring kingdom of Scotland.
In France political unity was reached only later and bit by bit, and when it came the difference of law in the various provinces was too firmly established to make uniformity possible until the time of the Revolution. In Germany the shadowy unity of the Holy Roman Empire was never enough to afford any effective central administration of justice. National law in the strict sense was impossible under such conditions: the most that can be expected is such a degree of unity as results from common traditions inherited from more primitive times, and a community of language and national feeling.
Amid local and national diversities of law there were at any rate two unifying influences, the Roman and the Canon law. In some parts of Europe, as in the South of France and Italy, the traditions of the Roman law had never died out, and in a debased form, with much admixture of the law of the invaders, it had come to form the basis of the local law. In others it was the barbarian law which formed the groundwork. But just as behind the new languages, whether in the main founded on Latin or on Teutonic, Latin remained the medium of intercourse between the countries of the West, and the instrument of thought and learning, so Roman law remained a tradition which was ever ready to exert an influence. It is not only in law courts that law is learnt and developed. Transactions have to be drawn up in writing, and will largely be made in Latin, and founded on precedents. The grants of land to and from ecclesiastical bodies especially will be in a form which borrows much from Roman or romanesque models; and they will form models for the transactions of others. Even the formulation of native law in the early codes will be carried out by men who know of no written law except the Roman. In the twelfth century Roman law becomes a subject of University study throughout Western Europe, in Italy, at Paris, even at Oxford, and forms a part of that international learning which scholars carry from land to land. Men trained in the Roman law rise to high positions in the public service. As judges and administrators they will not forget what they have learnt as students or taught as doctors. Yet it would be easy to exaggerate its influence, great as it was. It was certainly more as a form and method of legal thought than as an actual source of legal rules that it made itself felt, for instance, in our own country, and the strength and cohesion which it helped to give to our law enabled that law later to resist its further advance.
The Canon law was the law of the Western Church, a truly international society. It was formed largely on the model of the Roman law, and it largely borrowed from it, though it is full of non-Roman elements. It governed not merely what we should call purely ecclesiastical matters, but dealt or attempted to deal with other things, such as marriage and the disposition of the goods of the deceased. Our own law of marriage and divorce, and of probate of wills, has a history which goes back to the ecclesiastical law of the Middle Ages. Like the Roman law it exercised an influence as a model and a repository of maxims, all the greater because in every country it was a law in actual force within a sphere of which the boundaries were constantly being disputed between the lay and the church powers.
The beginnings of modern Europe with which we associate such things as the revival of learning and the Reformation brought with them on the Continent the event which is known as the reception of Roman law. The traditions of the ancient world had been seen in mediaeval times through mediaeval eyes, and had been moulded to mediaeval needs. The new age insisted on going back direct to the classical tradition. It was the actual Roman law of Justinian, not the Roman law as interpreted by mediaeval commentators, that was to be studied and applied. The break-up of the institutions of the Middle Ages, the growth of absolute monarchical power, the centralization of government, all favoured the tendency. Roman law contained doctrines eminently pleasing to an absolute ruler, e.g. 'the decision of the monarch has the force of law'. In Germany above all, where law was divided into countless local customs, the movement had its fullest effect. Roman law comes to be the law which is to be applied in the absence of positive enactment or justifiable custom. The native law finds itself driven to plead for its life, and is lucky if it can satisfy the conditions which are required to enable it to continue as a recognized custom. In every country of the West outside England, in greater or less degree, the Roman law comes in as something which will at least fill up the gaps, and will purge or remodel the native law. Even in Scotland texts of the Roman law may be quoted as authorities. The strength of our own law, and the successful resistance of our public institutions to monarchical power saved us alone from a 'reception', in the continental sense, of Roman law. And even our Blackstone will quote Roman law with respect where it tends to confirm our own rules.