Abstract

Natural law was a key concept in medieval moral and political theories. Originating in ancient Greece, it came to medieval thought mainly through the canon and civil law texts. Commentators on these texts tried to solve various difficulties, especially regarding the justification of property. Thomas Aquinas, Duns Scotus, William of Ockham, and many other theologians discussed natural law, especially in relation to God’s commandments.

In medieval texts the term ius naturale can mean either natural law or natural right; for the latter sense see the entry ‘‘Natural Right.’’ Ius naturale in the former sense, and also lex naturalis, mean the universal and immutable law to which the laws of human legislators, the customs of particular communities and the actions of individuals ought to conform. It is equivalent to morality thought of as a system of law. It is called ‘‘natural’’ either (a) because it is taught by natural instinct, that is, some capacity innate in human beings, or (b) because it is accessible to ‘‘natural reason,’’ that is, to personal reflection independent of any special revelation from God (such as the Christian faith claims to be) and independent of the moral authority of other human beings; or for both reasons.

Medieval writers referred to human law as ‘‘positive law,’’ ‘‘custom,’’ or ‘‘convention.’’ The positive law of a particular community was called ‘‘civil law’’ (there is no contrast here with criminal law); often the term referred especially to the civil law of the Romans. The term ‘‘law of nations’’ (ius gentium) seems equivalent in some Roman law texts to natural law, or perhaps to part of natural law; in medieval texts it usually means the laws common to the positive law of all, or most, human communities. (The boundary between natural law and the law of nations was never very clear.) Natural law was also distinguished from ‘‘divine positive law,’’ which is not knowable by natural reason but is notified to some human beings, and not to others, through special messengers (Moses, the Prophets, the Apostles, etc.).

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