The generation that came of age between the publication of Newton’s Principia in 1687 and the death of Louis XIV in 1715 tied the crucial knot between the Scientific Revolution and a new outlook on life. Whereas medieval and Reformation thinkers had been concerned primarily with abstract concepts of sin and salvation, and Renaissance humanists had drawn their inspiration from the classical past, Enlightenment thinkers believed that their era had gone far beyond antiquity and that intellectual progress was very possible. Talented writers of that generation popularized hard-to-understand scientific achievements and set an agenda of human problems to be addressed through the methods of science.

Like the Scientific Revolution, the Enlightenment was also fueled by Europe’s increased contacts with the wider world. In the wake of the great discoveries of the fifteenth and sixteenth centuries, the rapidly growing travel literature taught Europeans that the peoples of China, India, Africa, and the Americas all had their own very different beliefs and customs. Some Europeans began to look at truth and morality in relative, rather than absolute, terms. If anything was possible, who could say what was right or wrong?

The excitement of the Scientific Revolution also generated doubt and uncertainty, contributing to a widespread crisis in late-seventeenth-century European thought. In the wake of the devastation wrought by the Thirty Years’ War, some people asked whether ideological conformity in religious matters was really necessary. Others skeptically asked if religious truth could ever be known with absolute certainty and concluded that it could not. The atmosphere of doubt spread from religious to political issues. This was a natural extension, since many rulers viewed religious dissent as a form of political opposition and took harsh measures to stifle unorthodox forms of worship. Thus, questioning religion inevitably led to confrontations with the state.

These concerns combined spectacularly in the career of Pierre Bayle (1647–1706), a French Protestant, or Huguenot, who took refuge from government persecution in the tolerant 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 very influential Dictionary was found in more private libraries of eighteenth-century France than any other book.

Like Bayle, many Huguenots fled France for the Dutch Republic, a center of early Enlightenment thought for people of many faiths. The Dutch Jewish philosopher Baruch Spinoza (1632–1677) borrowed Descartes’s emphasis on rationalism and his methods of deductive reasoning, but he rejected the French thinker’s mind-body dualism. Instead, Spinoza came to believe 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 our actions were shaped by outside circumstances, not free will. Spinoza was excommunicated by the relatively large 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.

Out of this period of intellectual turmoil came John Locke’s Essay Concerning Human Understanding (1690). In this work, Locke (1632–1704), a physician and member of the Royal Society, brilliantly set forth a new theory about how human beings learn and form their ideas. Locke insisted that all ideas are derived from experience. The human mind at birth is like a blank tablet, or tabula rasa, on which the environment writes the individual’s understanding and beliefs. Human development is therefore determined by education and social institutions. 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. With his emphasis on the role of perception in the acquisition of knowledge, Locke provided a systematic justification of Bacon’s emphasis on the importance of observation and experimentation. The Essay Concerning Human Understanding passed through many editions and translations and, along with Newton’s Principia, was one of the dominant intellectual inspirations of the Enlightenment.