Natural law is the position that there is a moral order grounded in nature or in the structure of reality, knowable by reason without special revelation, and binding on everyone regardless of what any legal system happens to say.
Its lineage runs from the Stoics, who identified the natural law with the rational principle pervading the cosmos and drew from it the first clear statements of a universal moral community including slaves and foreigners; through Cicero, whose formulation, that true law is right reason in agreement with nature, unchanging and eternal, from which no senate or people can absolve us, became the standard classical citation; to Aquinas, whose treatment in the Summa Theologiae is the most influential in the Western tradition. Aquinas grounds natural law in the eternal law, gives as its first precept that good is to be done and pursued and evil avoided, and derives further precepts from natural inclinations: to self-preservation, to procreation and the raising of children, and to life in society and the knowledge of God.
Its practical consequences have been substantial and have run in more than one direction. Natural law arguments were used by Francisco de Vitoria and the School of Salamanca in the sixteenth century to argue that the indigenous peoples of the Americas held genuine rights and legitimate dominion, against those who denied it. They underlie the development of international law in Grotius, who argued that the law of nature would hold "even if we should concede that which cannot be conceded without the utmost wickedness, that there is no God". They inform the modern vocabulary of human rights, and they were invoked at Nuremberg against the defence that the acts charged had been lawful under German law. Martin Luther King Jr.'s Letter from Birmingham Jail cites Aquinas directly on the distinction between a just and an unjust law.
They are also the basis of Catholic moral teaching on contested questions, contraception, abortion, euthanasia, same-sex relationships, where the reasoning proceeds from ends held to be inherent in human faculties. This is where the tradition draws its sharpest criticism, and the objections are of several kinds: Hume's, that no "ought" follows from an "is"; the historical objection that appeals to "nature" have reliably encoded the assumptions of those making them; and the observation that natural law theorists tend to reach conclusions their own communities already hold, which suggests the reasoning is not doing the work claimed for it. Defenders reply that the "new natural law" theory of Germain Grisez and John Finnis reformulates the argument in terms of basic goods without deriving values from facts, and critics dispute that the reformulation succeeds.
Comparable positions exist well outside the Western tradition: the Mu'tazili claim that good and evil are knowable by reason, the Confucian appeal to the way of Heaven, and the Hindu conception of dharma as an order to be discerned rather than legislated.