Sources of the medieval concept of natural law

The chief sources on which the scholastics drew for their knowledge of natural law were Cicero, the Digest, St Paul, the Fathers and, later, Aristotle.

St Paul observed in his Epistle to the Romans, 2.12–16, that even without knowledge of the Old Testament Law pagans have its substance written on their hearts. Conscience and reason lead men to do by nature what the Law commands. Natural law thus accords with the Decalogue. Lactantius recorded Cicero's definition of law: true law is right reason in agreement with nature, being found among all men, summoning them to duty and prohibiting wrongdoing. True law may not be abolished by Senate or People; it is not different in Rome or in Athens, now or in the future. Its originator and promulgator is God; disobedience to it constitutes a denial of the nature of man.

The Digest in its first chapter distinguished three types of law: ius civile or the law of the state, ius gentium or the law of nations, and ius naturale or the law of nature. The jurists cited defined the natural law variously. Ulpian described it as the common instinct of animals; the union of male and female, the procreation of offspring and their education have been taught to animals by nature. But Gaius defined the natural law as those human laws practised by all nations and dictated to all men by natural reason, and Paulus said that the natural law consists of what is equitable and good.