Natural law, or the law of nature (Latin: lex naturalis), is a system of law that is purportedly determined by nature, and thus universal. Classically, natural law refers to the use of reason to analyze human nature — both social and personal — and deduce binding rules of moral behavior. Natural law is contrasted with the positive law (meaning “man-made law”, not “good law”; cf. posit) of a given political community, society, or nation-state, and thus serves as a standard by which to critique said positive law. According to natural law theory, which holds that morality is a function of human nature and reason can discover valid moral principles by looking at the nature of humanity in society, the content of positive law cannot be known without some reference to natural law (or something like it). Used in this way, natural law can be invoked to criticize decisions about the statutes, but less so to criticize the law itself. Some use natural law synonymously with natural justice or natural right.
Although natural law is often conflated with common law, the two are distinct in that natural law is a view that certain rights or values are inherent in or universally cognizable by virtue of human reason or human nature, while common law is the legal tradition whereby certain rights or values are legally cognizable by virtue of judicial recognition or articulation. Natural law theories have, however, exercised a profound influence on the development of English common law, and have featured greatly in the philosophies of Thomas Aquinas, Francisco Suárez, Richard Hooker, Thomas Hobbes, Hugo Grotius, Samuel von Pufendorf, John Locke, Francis Hutcheson, Jean Jacques Burlamaqui, and Emmerich de Vattel. Because of the intersection between natural law and natural rights, it has been cited as a component in United States Declaration of Independence and the Constitution of the United States, as well as in the Declaration of the Rights of Man and of the Citizen. Declarationism states that the founding of the United States is based on Natural law.