Natural law or the law of nature (Latin: lex naturalis) has been described as a law whose content is set by nature and that therefore has validity everywhere. As classically used, natural law refers to the use of reason to analyze human nature and deduce binding rules of moral behavior. The phrase natural law is opposed to the positive law (which is man-made) of a given political community, society, or nation-state, and thus can function as a standard by which to criticize that law. In natural law jurisprudence, on the other hand, the content of positive law cannot be known without some reference to the 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 (Latin ius naturale), although most contemporary political and legal theorists separate the two.

Natural law theories have 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. The essence of Declarationism is that the founding of the United States is based on Natural law.