Still more important was the antithesis between Nature and convention, which, so far as we know, originated exclusively with Hippias. We have already observed that universality and necessity were, with the Greeks, standing marks of naturalness. The customs of different countries were, on the other hand, distinguished by extreme variety, amounting sometimes to diametrical opposition. Herodotus was fond of calling attention to such contrasts; only, he drew from them the conclusion that law, to be so arbitrary, must needs possess supreme and sacred authority. According to the more plausible interpretation of Hippias, the variety, and at least in Greek democracies, the changeability of law proved that it was neither sacred nor binding. He also looked on artificial social institutions as the sole cause of division and discord among mankind. Here we already see the dawn of a cosmopolitanism afterwards preached by Cynic and Stoic philosophers. Furthermore, to discover the natural rule of right, he compared the laws of different nations, and selected those which were held by all in common as the basis of an ethical system.[63] Now, this is precisely what was done by the Roman jurists long afterwards under the inspiration of Stoical teaching. We have it on the high authority of Sir Henry Maine that they identified the _Jus Gentium_, that is, the laws supposed to be observed by all nations alike, with the _Jus Naturale_, that is, the code by which men were governed in their primitive condition of innocence. It was by a gradual application of this ideal standard that the numerous inequalities between different classes of persons, enforced by ancient Roman law, were removed, and that contract was substituted for status. Above all, the abolition of slavery was, if not directly caused, at any rate powerfully aided, by the belief that it was against Nature. At the beginning of the fourteenth century we find Louis Hutin, King of France, assigning as a reason for the enfranchisement of his serfs, that, ‘according to natural law, everybody ought to be born free,’ and although Sir H. Maine holds this to have been a mistaken interpretation of the juridical axiom ‘omnes homines naturâ aequales sunt,’ which means not an ideal to be attained, but a primitive condition from which we have departed: nevertheless it very faithfully reproduces the theory of those Greek philosophers from whom the idea of a natural law was derived. That, in Aristotle’s time at least, a party existed who were opposed to slavery on theoretical grounds of right is perfectly evident from the language of the _Politics_. ‘Some persons,’ says Aristotle, ‘think that slave-holding is against nature, for that one man is a slave and another free by law, while by nature there is no difference between them, for which reason it is unjust as being the result of force.’[64] And he proceeds to prove the contrary at length. The same doctrine of natural equality led to important political consequences,