criticism, regarded as potentially applicable to all men, in all ages, among all peoples, derived from reason and worked out philosophically. The proof of the ius gentium was in its acceptance and application as positive law among civilized peoples. The proof of ius naturale was in reason applied to the nature of things.
Roman jurists of the period of the strict law were little, if at all, affected by Greek philosophical ideas of natural law. What Cicero has to say about ius naturae38 is said from the standpoint of an orator and of a philosopher on the basis of Greek theories rather than of Roman law. In the classical period we hear much about natural law. But ‘the nature of things’ was thought of with reference to Roman legal institutions and legal conceptions, e.g., the nature of property and ownership and the nature of legal transactions, such as contract.39 Roman natural law seems a natural law drawn from idealized positive law, like what I have called ‘positive natural law’ as it was developed in American law in the nineteenth century.40 In the later empire Greek philosophical ideas affected the Greek teachers of law in the postclassical law schools and were applied to the classical texts.41 Yet the texts had repeatedly made the distinction that slavery exists by virtue of the ius gentium whereas slavery is contrary to ius naturale.42 Likewise they contrasted natural law with positive law in case of agnation and cognation.43
In Cicero we see the idea, which was to become strong in the Middle Ages and in the modern world, that ius naturale is the basis of all law and may not be set aside by the law of the state.44 The same idea appears frequently in the later law books.45 There was an ideal to which the law was to be shaped. But the law was not automatically abrogated by an appeal to natural law. Buckland puts the matter well: “The name ius naturale expresses a tendency in the trend of legal thought, a ferment which was operating all over the law.”46 It was not, however, a part of the positive law, as was the ius gentium. A pact might be binding by natural law and yet not create necessarily a naturalis obligatio in the positive law.47
The history of the modern science of law begins with the revival of the study of Roman law in the Italian universities of the twelfth century. From the twelfth to the seventeenth century there were two parallel lines of juristic development, one legal and the other philosophical. In the legal line of development the text of the Corpus Iuris was taken to be authoritatively binding. It could only be interpreted. But the texts spoke of reason, and in philosophy authority was held a ground of reason and church doctrine was declaratory of reason. As it was put by Erigena, the teachings of the fathers of the church, which rest on their authority, were discovered by them with the aid of reason.48 Anselm sought to prove the teachings of the Scriptures and of the fathers through reason so as to convince even the unbeliever.49 A revival of natural law came with study of the Institutes and Digest and was furthered by acceptance of Aristotle as authority in philosophy. Natural law was given a purely theological basis. But philosophy was turned to in order to reinforce theology and later was used as the foundation of a science of law which gradually cut loose from theology. Thus Lord Acton was moved to say that not the devil but St. Thomas Aquinas was the first Whig.
Thomas Aquinas (1225–1274) divided the old ius naturale into two: (1) lex aeterna, the eternal law, the “reason of the divine wisdom governing the whole universe,” and (2) lex naturalis, natural law, the law of human nature, proceeding ultimately from God but immediately from reason and governing the actions of men only. Positive law was a mere recognition of the lex naturalis which was above all human authority.50
It will be noted that Thomas Aquinas writes of lex naturae, not of ius naturale, or ius naturae. The law teaching of the medieval universities thought of law as legislation of the Emperor Justinian. It was not traditional in origin or in form. It was legislation, “proceeding from him who has the care of the community and promulgated.”51 The law imposed on Christendom by Justinian was analogous to the eternal law which the Creator had laid down for the universe. The lex naturae was the body of command imposed on mankind by the ultimate lawmaker, in part revealed and for the rest discoverable through reason.
It could be assumed that the legislation of the Christian emperor was declaratory of reason. But it was positive law, and appeal to the conscience against details of the positive law could be made in the name of natural law. The teaching of the Roman books as to the immutable precepts of natural law, beyond the reach of the lawmaker, was reinforced by theology. A precept contrary to natural law had no legal validity.52 But interpretation of and commentary on the text of the Corpus Iuris was enough for a general law for Continental Europe for three centuries before effective use could be made of the proposition. In England it came to be used to reinforce an idea of the reciprocal duties of lord and man when the king from the ultimate landlord was transformed into a governor ruling, as Sir Edward Coke told James I, under God and the law,53 and was used to uphold the authority of the church against encroachments by Parliament in Acts “impertinent to be observed,”54 from which in the right line of juristic descent we come to a fundamental doctrine of American law.55
The taught law of the medieval universities in Continental Europe was in a stage of strict law. As was said above, the Corpus Iuris was a body of legislation to be interpreted and applied. But there was no competent lawmaking authority in the polity of the time which could add to it or alter it. While the method of the academic expositors of Roman law was the scholastic method of formal logic applied to authoritative texts, like the Roman jurists of the republic, they were little influenced by philosophy. Hence the juristic function of natural law in the Middle Ages was limited. In legal history, when the balance of stability and change is overweighted on the side of stability, positive natural law is found organizing the body of stable precepts in their interpretation and application. When it is overweighted on the side of change we find natural natural law a directing agency of growth.
But theological natural law could operate indirectly through the canon law. What a good Christian would do and would not do gave an ideal that in one situation played a significant role. There were differences on moral grounds between the civilians and the canonists on the confines between law and morals.56 The civilians felt bound to adhere to the Roman doctrine that a bare pact was not legally enforceable and to argue that this was in accord with reason,57 while the canonists regarded a promise as binding on the conscience and so to be given legal effect on grounds of religion, natural law, and the practice of civilized peoples.58 Ultimately the canonist idea prevailed in Continental Europe. In the eighteenth century, Pothier, in the era of the law-of-nature school of jurists, explained that the Roman categories of enforceable pacts were very far from being in accord with nature and reason.59
We have seen that Greek philosophy gave Roman legal science two significant ideas: natural law or the law of nature, a universally valid body of ideal legal precepts, grounded in reason, and a body of legal precepts lying between natural law and the positive law of the particular state, on the one side more or less identical with natural law and on the other side received or recognized as positive law—a body of precepts common in varying degrees to the positive law of civilized peoples.
In England where from the thirteenth century the law was the law of the king’s courts and was taught by practicing lawyers in societies of lawyers and law students, lawyers did not trouble themselves about philosophy. Until the latter part of the sixteenth century or even till the seventeenth century English law was in the stage of strict law. In theory the courts applied the common custom of England. There was no such general body of custom common to all England—certainly not on many matters on which the courts had to pass. What the courts administered was a custom of judicial decision, not a custom of popular action. But it was the belief that judicial decision was ascertaining and declaring the established custom of the land that made it possible for the custom of decision to establish itself as law.
On the Continent, where the law that prevailed was a law of the universities, the academic law teachers were in immediate contact with theology and many of them were doctors of the canon law as well as of the civil law, jurisprudence was thought of as applied theology. The ideal element in law was supplied by a theological natural law. This mode of thought had some