In this essay, Alan Levine and Darren Staloff describe Enlightenment-era criticisms of the natural law tradition. One group of critics, the “naturalizers,” referred to natural law, but argued that it could be deduced from man’s natural state. For Voltaire, Montaigne, and Diderot, this was grounded in anthropological observations of Native Americans and Tahitians; for the contractarian naturalizers such as Hobbes, Locke, and Rousseau, rights were to be derived from how man would live in the “state of nature.” The second group of critics, the moral reductionists, rejected the prescriptive function of natural law, and based their reductive account of morality on psychological facts. For proponents of Epicureanism, such as Voltaire and Mandeville, the moral code was based on self-love and a desire for happiness and pleasure; for other reductionists – Shaftsbury, Hutcheson, Hume, and Smith – ethical knowledge was a sympathy to be cultivated, not a rational demonstration. Both types of reductionism were relativistic. The final group of critics described in this essay offered a prescriptive replacement of the natural law. For Rousseau the infallible “general will” served as the only source of legitimate authority. Mirabeau and Quesnay sought a descriptive and prescriptive source of normative judgment based on pre-civilized nature. Condillac and his followers, the sensationists, promoted a republican and liberal political order as the only way to achieve mankind’s necessary goals of avoiding pain and fulfilling desires. Enlightenment criticisms of natural law had profound implications for both political theory and practice, and, though the prescriptive replacements for natural law were the least philosophically radical in the intellectual realm, they proved to offer the most revolutionary political agenda in the practical realm, fueling the radical, and often violent, measures of the French Revolution.