The English philosopher and political theorist John Locke (1632-1704) laid much of the groundwork for the Enlightenment and made central contributions to the development of liberalism. Trained in medicine, he was a key advocate of the empirical approaches of the Scientific Revolution. In his “Essay Concerning Human Understanding,” he advanced a theory of the self as a blank page, with knowledge and identity arising only from accumulated experience. His political theory of government by the consent of the governed as a means to protect “life, liberty and estate” deeply influenced the United States’ founding documents. His essays on religious tolerance provided an early model for the separation of church and state.
The “Two Treatises of Government” (1690) offered political theories developed and refined by Locke during his years at Shaftesbury’s side. Rejecting the divine right of kings, Locke said that societies form governments by mutual (and, in later generations, tacit) agreement. Thus, when a king loses the consent of the governed, a society may remove him—an approach quoted almost verbatim in Thomas Jefferson’s 1776 Declaration of Independence. Locke also developed a definition of property as the product of a person’s labor that would be foundational for both Adam Smith’s capitalism and Karl Marx’s socialism.
In three “Letters Concerning Toleration” (1689-92), Locke suggested that governments should respect freedom of religion except when the dissenting belief was a threat to public order. Atheists (whose oaths could not be trusted) and Catholics (who owed allegiance to an external ruler) were thus excluded from his scheme. Even within its limitations, Locke’s toleration did not argue that all (Protestant) beliefs were equally good or true, but simply that governments were not in a position to decide which one was correct.