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Natural law is a system of law based on a close observation of natural order and human nature, from which values, thought by natural law's proponents to be intrinsic to human nature, can be deduced and applied independently of positive law. According to the theory of law called jusnaturalism, all people have inherent rights, conferred not by act of legislation but by "God, nature, or reason". Natural law theory can also refer to "theories of ethics, theories of politics, theories of civil law, and theories of religious morality". In Western tradition, natural law was anticipated by the pre-Socratics, for example, in their search for principles that governed the cosmos and human beings. The concept of natural law was documented in ancient Greek philosophy, including Aristotle, and was mentioned in ancient Roman philosophy by Cicero. References to it are also found in the Old and New Testaments of the Bible, and were later expounded upon in the Middle Ages by Christian philosophers such as Albert the Great and Thomas Aquinas. The School of Salamanca made notable contributions during the Renaissance. Although the central ideas of natural law had been part of Christian thought since the Roman Empire, its foundation as a consistent system was laid by Aquinas, who synthesized and condensed his predecessors' ideas into his Lex Naturalis. Aquinas argues that because human beings have reason, and because reason is a spark of the divine, all human lives are sacred and of infinite value compared to any other created object, meaning everyone is fundamentally equal and bestowed with an intrinsic basic set of rights that no one can remove. Modern natural law theory took shape in the Age of Enlightenment, combining inspiration from Roman law, Christian scholastic philosophy, and contemporary concepts such as social contract theory. It was used in challenging the theory of the divine right of kings, and became an alternative justification for the establishment of a social contract, positive law, and government—and thus legal rights—in the form of classical republicanism. John Locke was a key Enlightenment-era proponent of natural law, stressing its role in the justification of property rights and the right to revolution. In the early decades of the 21st century, the concept of natural law is closely related to the concept of natural rights and has libertarian and conservative proponents. Indeed, many philosophers, jurists and scholars use natural law synonymously with natural rights or natural justice; others distinguish between natural law and natural right. Some scholars point out that the concept of natural law has been used by philosophers throughout history also in a different sense from those mentioned above, e.g. for the law of the strongest, which can be observed
to hold among all members of the animal kingdom, or as the principle of self-preservation, inherent as an instinct in all living beings.