Another example may be seen in Aristotle's well-known discussion in the Nicomachean Ethics. It is significant that Greek thinkers always couple custom and enactment; things which today we contrast. These were the formal bases of legal authority. So Aristotle considers, not natural _law_ and positive _law_, but what is just in itself--just by nature or just in its idea--and what derives its sole title to be just from convention or enactment. The latter, he says, can be just only with respect to those things which by nature are indifferent. Thus when a newly reconstituted city took a living Spartan general for its eponymus, no one was bound by nature to sacrifice to Brasidas as to an ancestor, but he was bound by enactment and after all the matter was one of convention, which, in a society framed on the model of an organized kindred, required that the citizens have a common heroic ancestor, and was morally indifferent. The distinction was handed down to modern legal science by Thomas Aquinas, was embodied in Anglo-American legal thought by Blackstone, and has become staple. But it is quite out of its setting as a doctrine of _mala prohibita_ and _mala in se_. An example of the distinction between law and rules of law has become the basis of an arbitrary line between the traditionally anti-social, penalized by the common law, and recently penalized infringements of newly or partially recognized social interests. Although the discrimination between what is just and right by nature and what is just because of custom or enactment has had a long and fruitful history in philosophical jurisprudence and is still a force in the administration of justice, I suspect that the permanent contribution of Greek philosophy of law is to be found rather in the distinction between law and rules of law, which lies behind it and has significance for all stages of legal development. Roman lawyers came in contact with philosophy in the transition from the strict law to the stage of equity and natural law, and the contact had much to do with enabling them to make the transition. From a purely legal standpoint Greek law was in the stage of primitive law. Law and morals were still largely undifferentiated. Hence Greek philosophical thinking of a stage of undifferentiated law and morals lent itself to the identification of the legal and the moral in juristic thinking which was characteristic of the classical Roman law. But the strict law obviously was indifferent to morals and in many vital points was quite at variance with the moral ideas of the time. The Greek distinction of just by nature and just by convention or enactment was suggested at once by such a situation. Moreover the forms of law at the end of the Republic and at the beginning of the Empire invited a theory of law as something composite, made up of more than one type of precept and resting immediately on more than one basis of authority.