The term most often used for Enlightenment thinkers is philosophe, meaning simply "philosopher" in French. Many of the most famous and important philosophes were indeed French, but there were major English, Scottish, and Prussian figures as well. Some of the most noteworthy philosophes included the following.
John Locke: 1637 – 1704
Locke was an Englishman who, along with Newton, was among the founding figures of the Enlightenment itself. Locke was a great political theorist of the period of the English Civil Wars and Glorious Revolution, arguing that sovereignty was granted by the people to a government but could be revoked if that government violated the laws and traditions of the country. He was also a major advocate for religious tolerance; he was even bold enough to note that people tended to be whatever religion was prevalent in their family and social context, so it was ridiculous for anyone to claim exclusive access to religious truth.
Locke was also the founding figure of Enlightenment educational thought, arguing that all humans are born “blank slates” – Tabula Rasa in Latin – and hence access to the human faculty of reason had entirely to do with the proper education. Cruelty, selfishness, and destructive behavior were because of a lack of education and a poor environment, while the right education would lead anybody and everybody to become rational, reasonable individuals. This idea was hugely inspiring to other Enlightenment thinkers, because it implied that society could be perfected if education was somehow improved and rationalized.
Voltaire: 1694 – 1778
The pen name of François-Marie Arouet, Voltaire was arguably the single most influential figure of the Enlightenment. The greatest novelist, poet, and philosopher of France during the height of the Enlightenment period, Voltaire became famous across Europe for his wit, intelligence, and moral battles against what he perceived as injustice and superstition.
In addition to writing hilarious novellas lambasting everything from Prussia's obsession with militarism to the idiotic fanaticism of the Spanish Inquisition, Voltaire was well known for publicly intervening against injustice. He wrote essays and articles decrying the unjust punishment of innocents and personally convinced the French king Louis XV to commute the sentences of certain individuals unjustly convicted of crimes. He was also an amateur scientist and philosopher - he wrote many of the most important articles in the "official" handbook of the Enlightenment, the Encyclopedia (described below).
While he was a tireless advocate of reason and justice, It is also important to note the ambiguities of Voltaire's philosophy. He was a deep skeptic about human nature, despite believing in the existence and desirability of reason. He acknowledged the power of the ignorance and outmoded traditions to govern human behavior, and he expressed considerable skepticism that society could ever be significantly improved. For example, despite his personal disdain for Christian (especially Catholic) institutions, he noted that “if God did not exist, it would be necessary to invent Him,” because without a religious structure shoring up their morality, the ignorant masses would descend into violence and barbarism.
Emilie de Châtelet: 1706 - 1749
A major scientist and philosopher of the period, Châtelet published works on subjects as diverse as physics, mathematics, the Bible, and the very nature of happiness. Perhaps her best-known work during her lifetime was an annotated translation of Newton’s Mathematical Principles which explained the Newtonian concepts to her (French) readers. Despite the gendered biases of most of her scientific contemporaries, she was accepted as an equal member of the “republic of science.” In Châtelet the link between the legacy of the Scientific Revolution and the Enlightenment is clearest: while her companion (and lover) Voltaire was keenly interested in science and engaged in modest efforts at his own experiments, Châtelet was a full-fledged physicist and mathematician.
The Encyclopedia of Diderot and D’Alembert (1751)
The brainchild of two major French philosophes, the Encyclopedia was a full-scale attempt to catalog, categorize, and explain all of human knowledge. While its co-inventors, Jean le Rond D’Alembert and Denis Diderot, themselves wrote many of the articles, the majority were written by other philosophes, including (as noted above) Voltaire. The first volume was published in 1751, with other volumes following. In the end the Encyclopedia consisted of 28 volumes containing 60,000 articles with 2,885 illustrations. While its volumes were far too expensive for most of the reading public to access directly, pirated chapters ensured that its ideas reached a much broader audience.
The Encyclopedia was explicitly organized to refute traditional knowledge, namely that provided by the church and (to a lesser extent) the state. The claim was that the application of reason to any problem could result in its solution. It also attempted to be a technical resource for would-be scientists and inventors, not only describing aspects of science but including detailed technical diagrams of everything from windmills to mines. In short, the Encyclopedia was intended to be a kind of guide to the entire realm of human thought and technique - a cutting-edge description of all of the knowledge a typical philosophe might think necessary to improve the world.
David Hume: 1711 – 1776
Hume was the major philosopher associated with the Scottish Enlightenment, an outpost of the movement centered in the Scottish capital of Edinburgh. Hume was one of the most powerful critics of all forms of organized religion, which he argued smacked of superstition. To him, any religion based on "miracles" was automatically invalid, since miracles do not happen in an orderly universe knowable through science. In fact, Hume went so far to suggest that belief in a God who resembled a kind of omnipotent version of a human being, with a personality, intentions, and emotions, was simply an expression of primitive ignorance and fear early in human history, as people sought an explanation for a bewildering universe.
Hume also expressed enormous contempt for the common people, who were ignorant and susceptible to superstition. Hume is important to consider because he embodied one of the characteristics of the Enlightenment that often seems the most surprising from a contemporary perspective, namely the fact that it did not champion the rights, let alone anything like the right to political expression, of regular people. To a philosopher like Hume, the average commoner (whether a peasant or a member of the poor urban classes) was so mired in ignorance, superstition, and credulity that he or she should be held in check and ruled by his or her betters.
Adam Smith: 1723 - 1790
Smith was another Scotsman who did his work in Edinburgh. He is generally credited with being the first real economist: a social scientist devoted to analyzing how markets function. In his most famous work, The Wealth of Nations, Smith argued that a (mostly) free market, one that operated without the undue interference of the state, would naturally result in never-ending economic growth and nearly universal prosperity. His targets were the monopolies and protectionist taxes and tariffs that limited trade between nations; he argued that if states dropped those kind of burdensome practices, the market itself would increase wealth as if the general prosperity of the nation was lifted by an "invisible hand."
Smith's importance, besides founding the discipline of economics itself, was that he applied precisely the same kind of Enlightenment ideas and ideals to market exchange as did the other philosophes to morality, science, and so on. Smith, too, insisted that something in human affairs - economics - operated according to rational and knowable laws that could be discovered and explained. His ideas, along with those of David Ricardo, an English economist a generation younger than Smith, are normally considered the founding concepts of “classical” economics.
Jean-Jacques Rousseau (1712 - 1778)
Rousseau was the great contrarian philosophe of the Enlightenment. He rose to prominence by winning an essay contest in 1749, penning a scathing critique of his contemporary French society and claiming that its so-called “civilization” was a corrupt facade that undermined humankind’s natural moral character. He went on to write both novels and essays that attracted enormous attention both in France and abroad, claiming among other things that children should learn from nature by experiencing the world, allowing their natural goodness and character to develop. He also championed the idea that political sovereignty arose from the “general will” of the people in a society, and that citizens in a just society had to be fanatically devoted to both that general will and to their own moral standards (Rousseau claimed, in a grossly inaccurate and anachronistic argument, that ancient Sparta was an excellent model for a truly enlightened and moral polity). Rousseau’s concept of a moralistic, fanatical government justified by a “general will” of the people would go on to become of the ideological bases of the French Revolution that began just a decade after his death.