Loosely united by certain key questions and ideas, the European Enlightenment (ca. 1690–1789) was a broad intellectual and cultural movement that gained strength gradually and did not reach its maturity until about 1750. Its origins in the late seventeenth century lie in a combination of developments, including political opposition to absolutist rule, religious conflicts between Protestants and Catholics and within Protestantism, and the attempt to apply principles and practices from the Scientific Revolution to human society.

A key crucible for Enlightenment thought was the Dutch Republic, with its proud commitments to religious tolerance and republican rule. When Louis XIV demanded that all Protestants convert to Catholicism, around two hundred thousand Huguenots fled the country, many destined for the Dutch Republic. From this haven of tolerance, French Huguenots and their supporters began to publish tracts denouncing religious intolerance and suggesting that only a despotic monarch, not a legitimate ruler, would deny religious freedom. Their challenge to authority thus combined religious and political issues.

These dual concerns drove the career of one important early Enlightenment writer, Pierre Bayle (1647–1706), a Huguenot who took refuge from government persecution in the Dutch Republic. Bayle critically examined the religious beliefs and persecutions of the past in his Historical and Critical Dictionary (1697). Demonstrating that human beliefs had been extremely varied and very often mistaken, he concluded that nothing can ever be known beyond all doubt, a view known as skepticism. His influential Dictionary was found in more private libraries of eighteenth-century France than any other book.

The Dutch Jewish philosopher Baruch Spinoza (1632–1677) was a key figure in the transition from the Scientific Revolution to the Enlightenment. Deeply inspired by advances in the Scientific Revolution — and in particular by debates about Descartes’s thought — Spinoza sought to apply natural philosophy to thinking about human society. He borrowed Descartes’s emphasis on rationalism and his methods of deductive reasoning but rejected the French thinker’s mind-body dualism. Instead Spinoza came to espouse monism, the idea that mind and body are united in one substance and that God and nature were merely two names for the same thing. He envisioned a deterministic universe in which good and evil were merely relative values, and human actions were shaped by outside circumstances, not free will. Spinoza was excommunicated by the Jewish community of Amsterdam for his controversial religious ideas, but he was heralded by his Enlightenment successors as a model of personal virtue and courageous intellectual autonomy.

German philosopher and mathematician Gottfried Wilhelm von Leibniz (1646–1716), who had developed calculus independently of Isaac Newton, refuted both Cartesian dualism and Spinoza’s monism. Instead he adopted the idea of an infinite number of substances, or “monads,” from which all matter is composed according to a harmonious divine plan. His Theodicy (1710) declared that ours must be “the best of all possible worlds” because it was created by an omnipotent and benevolent God. Leibniz’s optimism was later ridiculed by the French philosopher Voltaire in Candide, or Optimism (1759).

Out of this period of intellectual turmoil came John Locke’s Essay Concerning Human Understanding (1690), perhaps the most important text of the early Enlightenment. In this work Locke (1632–1704) set forth a new theory about how human beings learn and form their ideas. Whereas Descartes based his deductive logic on the conviction that certain first principles, or innate ideas, are imbued in humans by God, Locke insisted that all ideas are derived from experience. According to Locke, the human mind at birth is like a blank tablet, or tabula rasa, on which understanding and beliefs are inscribed by experience. Human development is therefore determined by external forces, like education and social institutions, not innate characteristics. Locke’s essay contributed to the theory of sensationalism, the idea that all human ideas and thoughts are produced as a result of sensory impressions.

Along with Newton’s Principia, the Essay Concerning Human Understanding was one of the dominant intellectual inspirations of the early Enlightenment. Locke’s equally important contribution to political theory, Two Treatises of Civil Government (1690), argued that real sovereignty rested with an elected Parliament, not in the authority of the Crown.