Natural law is a system of law derived from nature and reason, not from legislation. It holds that there are objective moral principles that human laws should reflect. If a human law contradicts natural law, it is unjust and does not bind in conscience. The idea goes back to Aristotle and the Stoics, who argued that a rational order pervades the universe and that humans can discover it through reason. Thomas Aquinas systematized natural law in the 13th century, arguing that it is the participation of rational creatures in God's eternal law.

Natural law theory has been influential in ethics, politics, and jurisprudence. It grounds human rights, arguing that certain entitlements are inherent in human nature. It also grounds duties, arguing that certain acts are intrinsically wrong. Critics argue that natural law is vague, that it depends on controversial assumptions about human nature, and that it has been used to justify unjust institutions. Defenders reply that natural law provides a rational basis for morality that does not depend on revelation or convention. The debate continues in legal philosophy, bioethics, and political theory.

Key features of natural law

• Objective. Moral principles are discoverable by reason.
• Universal. They apply to all human beings.
• Normative. Human laws should conform to them.
• Teleological. They are grounded in human nature and purpose.

Natural law is not the same as the laws of nature, which describe how things behave. Natural law prescribes how things ought to be.