natural law, theory that some laws are basic and fundamental to human nature and are discoverable by human reason without reference to specific legislative enactments or judicial decisions. Natural law is opposed to positive law, which is human-made, conditioned by history, and subject to continuous change. The concept of natural law originated with the Greeks and received its most important formulation in Stoicism. The Stoics believed that the fundamental moral principles that underlie all the legal systems of different nations were reducible to the dictates of natural law. This idea became particularly important in Roman legal theory, which eventually came to recognize a common code regulating the conduct of all peoples and existing alongside the individual codes of specific places and times (see natural rights). Christian philosophers such as St. Thomas Aquinas perpetuated this idea, asserting that natural law was common to all peoples—Christian and non-Christian alike—while adding that revealed law gave Christians an additional guide for their actions. In modern times, the theory of natural law became the chief basis for the development by Hugo Grotius of the theory of international law. In the 17th cent., such philosophers as Spinoza and G. W. von Leibniz interpreted natural law as the basis of ethics and morality; in the 18th cent. the teachings of Jean Jacques Rousseau, especially as interpreted during the French Revolution, made natural law a basis for democratic and egalitarian principles. The influence of natural law theory declined greatly in the 19th cent. under the impact of positivism, empiricism, and materialism. In the 20th cent., such thinkers as Jacques Maritain saw in natural law a necessary intellectual opposition to totalitarian theories.
See J. Maritain, The Rights of Man and Natural Law (1943, repr. 1971); J. Fuchs, Natural Law (1965); J. Stone, Human Law and Human Justice (1965); A. Battaglia, Toward a Reformulation of Natural Law (1981).