Theories of natural law hold that there is a single law, or body of laws, based on nature, that all human societies should obey. This tradition, embraced by philosophers and legal theorists for more than 2,500 years, has been highly adaptable and multifarious.

The idea of natural law originated with the Greek philosopher Heraclitus (circa 500 B.C.), who declared, “All the laws of human beings are nourished by the one divine [law].” This universal principle was independent of human opinion or agreement, but rather was regarded as the justification for human laws. The corollary—that human ordinances are invalid if they conflict with the higher law—was asserted by the heroine of Antigone, where Sophocles has the heroine defy an edict on the grounds that “mortal man cannot transgress the gods’ unwritten and unfailing laws.” The Sophists too recognized this conflict when they pitted law (nomos) against nature (phusis). Law, they argued, was the result of human custom, agreement, and belief, and was therefore contingent, variable, and relative; but nature manifested itself in invariant instincts such as self‐​interest. Responding to this challenge, Plato contended that law was grounded in nature. He explicated this law in terms of his theory of Forms, the eternal principles of goodness and justice apprehended by reason rather than sense experience. Aristotle in the Rhetoric discussed the “law of nature” as an eternal, immutable principle, which was commonly invoked in Greek legal arguments. He elsewhere defended a notion of “natural justice,” variable but grounded in universal human nature.

The theory of natural law was expounded more fully by the Stoics, whose views were summarized by the Roman Cicero. “True law,” Cicero writes,

is right reason in agreement with nature, diffused among all men; constant and unchanging, it should call men to their duties by its precepts and deter them from wrongdoing by its prohibitions; and it never commands or forbids upright men in vain, while its rules and restraints are lost upon the wicked.… There will not be one law at Rome and another law at Athens, nor will there be a different law tomorrow than it is today; but one and the same law, eternal and unchangeable will bind all peoples and ages.

Natural law was divine in origin, but discoverable by human understanding: “This reason, when firmly fixed and fully developed in the human mind, is law.” This doctrine was endorsed by the early Christian writers, among them Lactantius.

Natural law was embraced by medieval philosophers, especially Thomas Aquinas, in his treatise on law (Summa Theologiae I-II, QQ. 90–97). He defined law (lex) generally as “an ordinance of reason for the common good which is made by the person who has care of the community, and which is promulgated.” Aquinas distinguished four principal types of law. Eternal law is the principle by which God rationally governs the universe. Natural law is “the sharing in the eternal law by rational creatures.” They can employ their reason to serve their God‐​given natural ends. Human or positive law is rationally derived from natural law. “If at any point it deflects from the law of nature, it is no longer a law but a perversion of law.” Finally, divine (positive) law is laid down by God, but, unlike eternal law, it is revealed through faith, rather than by reason. Aquinas held that natural law, apprehended by reason, concurs with divine law, as revealed to the Christian church. This concord remained the core of traditional natural law theory, although it was elaborated by later Scholastic and neo‐​Thomist thinkers.

Many modern philosophers repudiated this scholastic tradition, including Baruch Spinoza. “Nothing is absolutely prohibited by the law of nature,” Spinoza maintained, “unless it is physically impossible.” However, there have been modern advocates of natural law, the most important of which were Hugo Grotius (1583–1645) and Samuel Pufendorf (1632–1694). Grotius argued that natural law was the foundation for international law and the law of war and peace. It was “a dictate of right reason, which posits that an act, according as it is or is not in conformity with rational nature, has in it a quality or moral baseness or moral necessity, and that, in consequence, such an act is either forbidden or enjoined by the author of nature, God.” Grotius claimed that his theory would be valid, “even if we concede what which cannot be conceded without the utmost wickedness, that there is no God, or that the affairs of men are of no concern to Him.”

The most influential modern theorist, John Locke, declared in his Second Treatise of Government that “the State of Nature has a Law of Nature to govern it, which obliges every one: And reason, which is that law, teaches all mankind, who will but consult it, that being all equal and independent, no one ought to harm another in his life, health, liberty, or possessions.” Locke affirmed the divine origin of natural law: “For all men being all the workmanship of one omnipotent, and infinitely wise maker; all the servants of one sovereign master, sent into the world by his order and about his business, they are his property, whose workmanship they are, made to last during his, not another’s pleasure.” According to the law of nature,

Every one as he is bound to preserve himself, and not to quit his station willfully; so by the like reason when his own preservation comes not in competition, ought he, as much as he can, to preserve the rest of mankind, and may not unless it be to do justice on an offender, take away, or impair the life, or what tends to the preservation of the life, liberty, health or goods of another.

Natural law is thus the foundation of man’s rights to life, liberty, and property. The positive laws of governments “are only so far right as they are founded on the Law of Nature, by which they are to be regulated and interpreted.” Locke influenced the American founders, who invoked “the Laws of Nature and of Nature’s God” in the American Declaration of Independence.

Aquinas and Locke represent opposing poles of natural law theory: traditionalist versus modernist. Traditional theorists emphasize moral duties and obedience to political authorities. They condemn abortion, homosexual unions, and contraception as unnatural acts, and they support governmental enforcement of morality. Modernist theorists are generally more libertarian and advocate revolution to protect natural rights. For Locke, the right to liberty was “the foundation of all the rest” and “liberty is to be free from restraint and violence from others.” For traditionalists like Lord Acton and Pope John Paul II, “freedom consists not in doing what we like, but in having the right to do what we ought.” Twentieth‐​century theorists also have disagreed over whether democratic capitalism or socialism is more in accord with natural law. Recent proponents of natural law jurisprudence include Lon Fuller, John Finnis, and Michael Moore.

Further Readings

Finnis, John. Natural Law and Natural Rights. Corr. ed. Oxford: Clarendon Press, 1988.

Fuller, Lon. The Morality of Law. Rev. ed. New Haven, CT: Yale University Press, 1969.

George, Robert P., ed. Natural Law Theory. Oxford: Clarendon Press, 1992.

Hittinger, Russell. A Critique of the New Natural Law Theory. Notre Dame, IN: University of Notre Dame Press, 1987.

Lisska, Anthony J. Aquinas’s Theory of Natural Law: An Analytic Reconstruction. Oxford: Clarendon Press, 1996.

Miller, Fred D., Jr. “Aristotle on Natural Law and Justice.” A Companion to Aristotle’s Politics. David Keyt and Fred D. Miller, Jr., eds. Oxford: Blackwell, 1991.

Moore, Michael. “A Natural Law Theory of Interpretation.” Southern California Law Review 58 (1985): 277–398.

Paul, Ellen, Fred D. Miller, Jr., and Jeffrey Paul, eds. Natural Law and Modern Moral Philosophy. Cambridge: Cambridge University Press, 2001.

Rommen, Heinrich A. The Natural Law: A Study in Legal and Social History and Philosophy. Thomas R. Hanley, trans. Indianapolis, IN: Liberty Press, 1998. [orig. German 1936]

Veatch, Henry B. Human Rights: Fact or Fancy? Baton Rouge: Louisiana State University Press, 1985.

Originally published