Key Thinkers of the Enlightenment: Voices That Shaped the Modern World

The Enlightenment was the work of individuals, not of a “Spirit of the Age” any more cooperative than the spirit of any other age. Behind the great movements, the revolutionary manifestos, and the political upheavals stood a community of writers, philosophers, scientists, and reformers whose ideas, arguments, and — let us admit — vanities and feuds shaped the era. The thinkers of the eighteenth century remain strikingly modern in their concerns: how should political power be organized and limited, what is the basis of moral judgment, how should economies be governed, and what are the rights and responsibilities of individuals in society.

The selection below is partial. The eighteenth century produced thousands of significant writers, and the choices made here reflect the period’s later influence rather than its contemporary reach. Some of these figures — Voltaire, Smith, Kant — were the celebrities of their day. Others — Spinoza, Wollstonecraft, Beccaria — spent long stretches in obscurity before the nineteenth and twentieth centuries decided they mattered. Both kinds of attention are part of the history.

The French Philosophes

No group of thinkers had a more decisive influence on the public culture of the Enlightenment than the French philosophes. Concentrated in Paris, working through the great salons and the great publishing projects of the century, they gave the Enlightenment its most characteristic voice. They were also, in their own time, considerably more quarrelsome than the canon makes them look. Diderot and Rousseau had a famous public falling-out in 1757 that was, in effect, a divorce performed in print. Voltaire and Rousseau never forgave each other anything. The salons, far from being havens of polite agreement, were competitive arenas where reputation was the principal currency and where an unlucky remark could end a career.

Voltaire (1694–1778)

Voltaire, born François-Marie Arouet, was the most famous writer of the eighteenth century and the most influential single figure of the French Enlightenment. A prolific author of tragedies, histories, philosophical tales, pamphlets, letters, and dictionaries, Voltaire combined literary brilliance with polemical force. He was a tireless campaigner against religious intolerance, arbitrary power, and the cruelty of the criminal justice system.

His works, including the Letters on the English Nation (1733) — originally published in English as Letters Concerning the English Nation and then, when the French edition came out, immediately suppressed and publicly burned — the Philosophical Dictionary (1764), and the satirical novel Candide (1759), brought Enlightenment ideas to a vast European readership. Candide sold more copies in its first year than most eighteenth-century novels managed in a decade. The famous cry “Écrasez l’infâme” — “Crush the infamous thing” — was his lifelong battle against superstition and clerical power.

Voltaire was not a systematic philosopher. He was a writer of great intelligence and even greater wit, and his influence came above all through style. To be “Voltairean” in the eighteenth century meant to be skeptical, irreverent, and committed to the cause of reason and justice. He was also capable of being wrong in interesting ways: his late enthusiasm for the English poet and foreman of a watch-trade John Law’s Mississippi Scheme is a reminder that the greatest minds of the age could be just as credulous as the lesser ones, given the right circumstances. Worth knowing about.

Jean-Jacques Rousseau (1712–1778)

Rousseau was the most original and the most troubling of the great French thinkers. Born in Geneva, trained in no systematic school, and possessed of a deeply uneasy relationship with the Parisian salons, Rousseau became famous with his Discourse on the Arts and Sciences (1750), which won an essay prize from the Academy of Dijon by arguing that the progress of the arts and sciences had not improved human morality. The academicians were not pleased. The reading public was.

This contrarian move was characteristic. Rousseau went on to write the Discourse on the Origin of Inequality (1755), the novel Julie, or the New Heloise (1761), the educational treatise Émile (1762), and above all the Social Contract (1762), whose opening line — “Man is born free, and everywhere he is in chains” — became a watchword of the revolutionary age. The Parliament of Paris condemned Émile in June 1762 and ordered Rousseau arrested; he fled to Switzerland, then to Neuchâtel, then to England with Hume (the two fell out spectacularly), and finally back to France under an assumed name. Émile was a sensation; Rousseau’s life became the model for the Romantic notion of the persecuted genius.

Rousseau’s thought was deeply influential on the French Revolution and on Romanticism. His theory of the general will, his account of natural man, and his critique of modern society provoked arguments that have not been resolved. The general will is, in particular, the most argued-over idea in modern political thought: it is at once a noble conception of civic participation and a dangerous doctrine that can be used to justify the suppression of dissent in the name of the collective. Robespierre claimed to be reading Rousseau when he signed the death warrants. So did the Girondins whom Robespierre sent to the scaffold. The text does not resolve the question; that is its enduring power.

Montesquieu (1689–1755)

Montesquieu, born Charles-Louis de Secondat, Baron de La Brède et de Montesquieu, was one of the founders of modern comparative politics and sociology. His masterwork, The Spirit of the Laws (1748), examined the laws, customs, and political institutions of different societies in a systematic way and argued that political forms were shaped by climate, geography, commerce, and history. The book was put on the Index of Forbidden Books by the Sorbonne within a year of publication, which is usually a sign that the Sorbonne had not read it carefully.

Montesquieu is best known today for his theory of the separation of powers, which proposed that legislative, executive, and judicial functions should be distributed among different branches of government. This theory had an enormous influence on the framers of the American Constitution. The US founders had a tendency, common in the eighteenth century, to read Montesquieu’s description of the English constitution as a description of how the English constitution actually worked, which it did not; England had, and has, no formal separation of powers in the American sense. The lesson is worth keeping in mind: the founding documents of modern liberalism are full of empirical errors. They are nevertheless the founding documents.

Denis Diderot (1713–1784)

Diderot was the principal editor of the Encyclopédie, the great collaborative project that aimed to summarize all human knowledge. He was also a novelist, playwright, art critic, and natural philosopher of exceptional range. His works included the novel Jacques the Fatalist (written 1765–1780, published 1796), the dialogue Rameau’s Nephew (written c. 1761–1774), and a vast body of writings on science, aesthetics, and political reform. Most of his most daring works — the dialogue Le Neveu de Rameau, the philosophical Rêve de d’Alembert, the sexually explicit Les Bijoux indiscrets — were published only after his death. He was, in this respect, a martyr to his own prudence.

Diderot was committed to a materialist philosophy that gave little room to traditional religious belief. He visited Russia at the invitation of Catherine the Great in 1773, expecting to build a model Enlightenment state; he returned within a year, having decided that Catherine wanted a philosopher as a mascot, not a partner. The episode is the eighteenth-century equivalent of a Western academic being invited to advise a dictatorship on human rights.

Other French Voices

Beyond the four giants, the French Enlightenment included many other influential writers. The Baron d’Holbach (1723–1789) published the System of Nature (1770), a comprehensive statement of atheistic materialism — the book was publicly burned by the Paris hangman, which made it the most-discussed philosophical work of 1771. Helvétius (1715–1771), in On Mind (1758), argued that all intellectual differences were the result of education; the Sorbonne condemned the book, the Pope put it on the Index, and Frederick the Great offered Helvétius a pension. The Marquis de Condorcet (1743–1794) wrote on mathematics, politics, and the idea of progress, and died in a French prison during the Terror, possibly by his own hand. The Marquis de Sade (1740–1814) took Enlightenment materialism in dark and transgressive directions, was imprisoned for most of his adult life, and wrote his best works in the Bastille and the Charenton asylum. The Abbé Sieyès (1748–1836) wrote the influential pamphlet What Is the Third Estate? (1789) — “What is the Third Estate? Everything. What has it been hitherto in the political order? Nothing. What does it want? To become something” — which helped launch the French Revolution and which survives, even now, as one of the great political opening salvos of the modern world.

The British Empiricists

The British tradition of empiricism provided a powerful alternative to French rationalism, and its influence on the Enlightenment was enormous. It was also, like every other national tradition in this period, deeply dependent on Continental imports. Locke spent his formative years in exile in Holland; Hume lived for three years in France; Smith spent most of his later life in the company of French economists. The British Enlightenment is a national tradition only in the sense that the men who did the writing happened to live in Britain.

John Locke (1632–1704)

John Locke was the most important political philosopher of the late seventeenth century and one of the founding figures of the modern liberal tradition. His Essay Concerning Human Understanding (1689) argued that all human knowledge derived from experience, and his Two Treatises of Government (also 1689) provided a systematic defense of revolution against tyranny and an early statement of the doctrine of natural rights.

The Essay is, in its origins, an attack on the doctrine of innate ideas. Locke’s argument is, roughly, that if there were innate ideas, they would be available to all human beings at all times, which they are not. The argument is more interesting than it sounds; it was directed against a particular school of Cambridge Platonists, but it became the foundation of the empirical tradition that culminated in Hume. The Two Treatises were written in the late 1670s and 1680s as a defense of the exclusion of James, Duke of York, from the throne, and then revised as a defense of the Glorious Revolution of 1688. They have done more political work than any other work of modern political philosophy.

Locke was also an important theorist of religious toleration. His Letter Concerning Toleration (1689) argued that the state should not use force in matters of religious belief, and that the church was a voluntary association of believers with no right to coerce its members or non-members. The argument is genuinely important, and it is also genuinely limited: Locke excluded Catholics (because they owed allegiance to a foreign sovereign) and atheists (because they could not be trusted to keep promises on the Day of Judgment) from the scope of toleration. The exclusions are embarrassing to a modern reader, and they should be embarrassing. They are also typical of the period. The full extension of toleration to all groups was a long process that took another two centuries.

David Hume (1711–1776)

David Hume was the most formidable skeptical philosopher of the Enlightenment. His Treatise of Human Nature (1739–1740) attempted to apply the experimental method of natural science to the study of human nature, with results that shocked his contemporaries. Hume argued that causation was not a matter of logical necessity but of habitual association, that personal identity was a fiction constructed by the imagination, and that the foundations of morality were to be found in sentiment rather than reason. “Reason is, and ought only to be, the slave of the passions,” he wrote, a sentence that has been quoted as often as it has been understood.

The Treatise sold so badly that Hume later called it a stillborn child. He reworked his arguments into the shorter and more accessible Enquiry Concerning Human Understanding (1748) and Enquiries Concerning the Principles of Morals (1751). His Dialogues Concerning Natural Religion (1779) applied his skeptical methods to arguments for God’s existence and helped undermine natural theology in its classical form; the book was published posthumously, three years after his death, because no London printer would put his name to it during Hume’s lifetime. The posthumous publication was arranged by Adam Smith, who also arranged for Hume to be buried in a Calvinist kirkyard in Edinburgh — Hume being a notorious infidel — and who, in a touching letter, reported that his friend had faced his own approaching death “with great cheerfulness.”

Hume’s influence on later philosophy was enormous. He provoked the responses of Kant and the German Idealists. He influenced the utilitarian tradition in Britain. His skeptical arguments are still debated in contemporary philosophy.

Adam Smith (1723–1790)

Adam Smith was the founder of modern economics. A close friend of Hume, Smith held the chair of moral philosophy at the University of Glasgow before retiring to write his two great books. The Theory of Moral Sentiments (1759) examined the psychological foundations of morality, arguing that human beings were equipped with a natural capacity for sympathy. The Wealth of Nations (1776) laid the foundations of classical political economy, arguing that markets, when free from regulation and monopoly, tended to allocate resources efficiently.

Smith was no defender of unregulated capitalism in the modern sense. He was a critic of monopoly, of corporate power, and of the excessive influence of merchants and manufacturers on government — the famous passage about the “mean rapacity” of businessmen who “neither are, nor ought to be, the rulers of mankind.” The “invisible hand” appears only three times in all of his writings, and never in the Wealth of Nations. Smith’s economics cannot be separated from his moral philosophy; the “impartial spectator” of the Theory of Moral Sentiments is meant to keep the market’s self-interest from running amok. The twentieth-century cult of Smith as a prophet of self-interest is, in this respect, a misreading — though it is a misreading that Smith himself has to bear some responsibility for, given that he sometimes wrote as if the pursuit of self-interest needed no further justification.

Other British Voices

The British Enlightenment included many other important thinkers. The Scottish philosopher Francis Hutcheson (1694–1746) was a forerunner of Smith and a defender of the moral sense theory; he was, by all accounts, a much better teacher than a writer. The historian Edward Gibbon (1737–1794) wrote The History of the Decline and Fall of the Roman Empire (1776–1789), a masterpiece of Enlightenment historiography whose famous fifteenth and sixteenth chapters — on the rise of Christianity — provoked a public outcry that has never quite stopped. The conservative political thinker Edmund Burke (1729–1797), author of Reflections on the Revolution in France (1790), became the great critic of the radical Enlightenment. The utilitarian philosopher Jeremy Bentham (1748–1832) extended Enlightenment rationalism into a systematic program of legal and political reform, beginning with the famous objection that the rights of man were “nonsense upon stilts.” The literary figures Joseph Addison (1672–1719) and Richard Steele (1672–1729) shaped public taste and public opinion through their periodicals The Tatler and The Spectator; the two were the original Twitter-feeds, in the sense that they produced short, witty, often anonymous pieces for a wide audience.

The German Aufklärung

The German Enlightenment, or Aufklärung, was distinct from the French and British versions. More rooted in universities, more given to systematic philosophy, and more concerned with questions of aesthetics and education, the German Enlightenment produced an extraordinary body of work that culminated in the philosophy of Immanuel Kant. It was also, by the late eighteenth century, more deeply read across Europe than the British, and as philosophically ambitious as the French. The minor-key tone of the Aufklärung — the Gemütlichkeit, the long dinners, the professorial jokes — has concealed, for too long, how radical it actually was.

Immanuel Kant (1724–1804)

Kant spent his entire life in Königsberg, in East Prussia, and his philosophy represented the highest point of the German Enlightenment. His three great Critiques — the Critique of Pure Reason (1781), the Critique of Practical Reason (1788), and the Critique of Judgment (1790) — defined the modern problem of knowledge, established a new foundation for morality, and proposed a theory of aesthetic judgment.

Kant’s practical philosophy, especially the Groundwork of the Metaphysics of Morals (1785), argued that morality was grounded in reason rather than in the consequences of actions or in human sentiment. His famous categorical imperative — “Act only according to that maxim by which you can at the same time will that it become a universal law” — gave philosophical expression to the Enlightenment commitment to universal moral principles. The argument is austere, abstract, and easy to parody; it has also been, for two and a half centuries, the most discussed foundation for deontological ethics.

His essay What Is Enlightenment? (1784) defined the Enlightenment as “man’s emergence from self-imposed immaturity” and became the canonical statement of Enlightenment ideals. Notice the word “self-imposed”: Kant’s argument is not that we are kept in immaturity by kings or priests, but that we are kept in immaturity by our own fear of thinking for ourselves. That is a darker and more accurate diagnosis than the one usually offered. His essay on Perpetual Peace (1795) is the founding document of modern internationalism, even if the federation of free republics that Kant imagined still does not exist.

Other German Thinkers

Gotthold Ephraim Lessing (1729–1781) was a dramatist, critic, and philosopher whose plays, including Nathan the Wise (1779), defended religious toleration. Lessing was the great popularizer of the German Enlightenment; he made the new ideas accessible to a wide German public, and he created the cultural space in which Kant’s Critiques could be received. Moses Mendelssohn (1729–1786), the grandfather of the composer, was a philosopher who defended the rational core of Judaism and contributed to the theory of aesthetics; his Jerusalem (1783) is the founding document of modern Jewish political philosophy. Christian Wolff (1679–1754) was a systematic philosopher whose rationalist works influenced the German university tradition; he was expelled from Halle in 1723 on a charge of atheism (the charge was, in effect, for teaching that the orderliness of the universe could be explained by reason alone), and he was recalled in triumph by Frederick the Great in 1740. The historian and jurist Samuel Pufendorf (1632–1694) and the philosopher Christian Thomasius (1655–1728) were important forerunners of the German Aufklärung.

The American Founders

The American Enlightenment is sometimes treated as a provincial offshoot of the European movement, but the work of the American founders was creative in its own right and enormously influential. The American case is also the test case for what the Enlightenment looks like when it has to govern a real country, with all the mess that implies.

Thomas Jefferson (1743–1826)

Thomas Jefferson, the principal author of the Declaration of Independence and the third President of the United States, was a deeply read student of the European Enlightenment. His Declaration drew on Locke’s political philosophy, his Notes on the State of Virginia (1785) was a model of Enlightenment natural history, and his letters to correspondents across the Atlantic kept him in touch with the latest European ideas.

Jefferson was also a complex figure, a slaveholder who wrote that “all men are created equal” and who, at the time of his death, was responsible for the freedom of only a small fraction of the people he had enslaved. The contradiction is not a footnote. It is the central fact of his life, and the failure to acknowledge it is a failure to understand the period. The Declaration was a manifesto; the slave quarter at Monticello was a counter-manifesto. Both are part of the same man.

Benjamin Franklin (1706–1790)

Benjamin Franklin was the most famous American of his age. A printer, scientist, inventor, diplomat, and political leader, Franklin was a man of the Enlightenment in every sense. His Poor Richard’s Almanack (1732–1758) made him rich and famous; his scientific experiments, especially on electricity (the famous kite experiment of 1752, which probably did not happen the way it is usually described), made him an international celebrity; and his role as a diplomat during the American Revolution and the early years of the Republic showed him to be a master of the political arts. He was also a good writer in an age of good writers, which is the highest praise an eighteenth-century American could hope for.

Franklin’s Autobiography is a founding text of American literature, and his life remains a model of the Enlightenment idea of the self-made intellectual. It is also, in retrospect, a more equivocal text than it was once taken to be: the cheerful image of the self-made man covers a long career of self-promotion that Franklin’s own letters disclose.

Thomas Paine (1737–1809)

Thomas Paine was an Englishman who became an American and then a French citizen, and one of the eighteenth century’s most spectacularly unsuccessful politicians. His pamphlet Common Sense (1776) did as much as any single work to galvanize American public opinion in favor of independence; it was, by one estimate, read by almost every literate adult in the thirteen colonies. His later Rights of Man (1791–1792) was a systematic defense of republican government against the attacks of Edmund Burke, and The Age of Reason (1794–1807) was a freethinking critique of organized religion that alienated many of the Americans who had previously been his strongest supporters.

Paine was the most radical of the major Enlightenment figures on both sides of the Atlantic, and his writings remained an inspiration to popular political movements throughout the nineteenth century. He was also, by the end of his life, an alcoholic and a social outcast; he died in Greenwich Village in 1809, and only six people attended his funeral. The political establishment of the early Republic did everything possible to bury his memory. The labor movements of the nineteenth century did everything possible to keep it alive. Both reactions are part of the meaning of Paine.

Other Important Figures

Cesare Beccaria (1738–1794)

Cesare Beccaria, an Italian jurist, wrote On Crimes and Punishments (1764), one of the most influential works of legal reform in modern history. The book is short — barely a hundred pages — and reads more like a manifesto than a treatise. Beccaria argued against torture, against capital punishment, and against arbitrary judicial procedure, and he proposed a theory of punishment based on deterrence and proportion rather than vengeance. His work was translated throughout Europe and had a direct influence on the criminal law reforms of the late eighteenth and nineteenth centuries. Catherine the Great was reading Beccaria in the year the Nakaz was published; the Marquise du Châtelet translated him; Voltaire praised him. The book is also, in its quiet way, one of the founding documents of the modern liberal state, and it remains a much better book than its reputation suggests.

Mary Wollstonecraft (1759–1797)

Mary Wollstonecraft is the most important feminist thinker of the Enlightenment. Her A Vindication of the Rights of Woman (1792) extended the logic of the French Declaration of the Rights of Man to women, and argued that women were not naturally inferior to men but appeared so only because they lacked education. The argument is, in the context of 1792, astonishingly aggressive. She is not asking for a more equitable share of the existing order; she is arguing that the existing order, including the way it constructs “femininity,” is a kind of systematic fraud.

Her other works — the novel Maria: or, The Wrongs of Woman (1798), her travel writing, and her letters — established the foundations of modern feminist thought. Wollstonecraft died at the age of thirty-eight, having experienced an extraordinarily full life, and her influence was carried forward by her daughter Mary Shelley, the author of Frankenstein (1818). It was also carried forward, often in spite of itself, by a male canon that responded to her arguments by patronizing her, and that did not, in most of the English-speaking world, take her seriously until well into the twentieth century.

Baruch Spinoza (1632–1677)

Spinoza was a Dutch-Jewish philosopher of Portuguese origin whose Ethics (1677) and Theological-Political Treatise (1670) presented a thoroughgoing rationalist account of God, nature, and human freedom. The Theological-Political Treatise was published anonymously, and the author was widely suspected of being an atheist; the Ethics was published, after his death, in 1677 with Spinoza’s name on the title page and was, almost immediately, placed on the Index. Spinoza is, of all the canonical figures of the Enlightenment, the one most likely to be left out of the standard accounts; he died in 1677, before the period usually called the Enlightenment began. He is also, on Jonathan Israel’s argument, the central figure of the entire period, the founder of the Radical Enlightenment, and the source of everything that matters in modern secular thought. The dispute is unresolved; both readings have merit.

Spinoza’s influence on later thought was enormous, although it operated for a long time beneath the surface. He influenced the German Idealists, the Romantics, and, in the twentieth century, the philosophers of secularism and liberal democracy. He has, in particular, been rediscovered in the last fifty years as the most interesting political philosopher of the seventeenth century — perhaps of any century. He is overpraised by some, underread by most, and worth the time it takes.

How the Thinkers Influenced Each Other

The Enlightenment was a conversation, and the conversation was conducted through books, letters, translations, and personal encounters. Voltaire visited England and wrote enthusiastically about English constitutionalism; Rousseau was personally received by Hume in Paris and later took refuge in England; Diderot visited Catherine the Great in Saint Petersburg; Franklin and Jefferson spent years in Paris and knew the philosophes personally.

The result was a constant cross-fertilization of ideas. The American founders read Locke and Montesquieu, the French philosophes read Newton and Hume, the German Aufklärer read everyone and tried to synthesize the whole tradition. This is why the Enlightenment is best understood not as a collection of national movements but as a single, multilingual, often contentious, and always vibrant conversation. Worth pausing on: the cross-currents are not just at the level of ideas but at the level of institutions. The Académie des sciences in Paris, the Royal Society in London, the Berlin Academy, the Saint Petersburg Academy, the provincial academies in Bordeaux and Toulouse — all of these were connected by letter, by exchange of members, and by the same small set of languages (Latin, French, increasingly English and German) in which the conversation could be carried on.

The Thinkers and Their Critics

The thinkers of the Enlightenment were not without their critics, even within their own ranks. Rousseau famously questioned whether the arts and sciences had improved human morality, and his critique of modern society has never been fully answered. The German Romantics rejected the abstract rationalism of the French and British traditions in favor of history, language, and feeling. The conservative Burke defended tradition, prescription, and prejudice — using the term in its original sense of pre-judgment — against the abstract universalism of the revolutionaries.

In the twentieth and twenty-first centuries, the critics of the Enlightenment have been even more vocal. The Frankfurt School found in the Enlightenment the seeds of modern totalitarianism. Postcolonial critics have seen in it a cover for European domination. Feminists have pointed to the way in which the supposedly universal language of rights excluded women. Environmentalists have questioned its commitment to the domination of nature.

These critiques have not displaced the thinkers of the Enlightenment; they have, rather, complicated our understanding of them. The contemporary heirs of the Enlightenment embrace its commitments to reason, science, and human rights while accepting its limits and blind spots. The thinkers of the eighteenth century remain essential reading, both for what they got right and for what they got wrong. The honest position is to read them for both at the same time.

Conclusion

The thinkers of the Enlightenment — from Voltaire and Rousseau to Locke and Hume, from Kant and Lessing to Jefferson and Wollstonecraft — produced a body of work that continues to shape the modern world. Their arguments about reason, liberty, equality, and human dignity are still the moral vocabulary of contemporary politics, even when contemporary politics is unsure whether to use them. Their methods of inquiry — empirical, critical, willing to question authority — remain the foundation of modern science. Their political experiments — constitutional government, the protection of individual rights, the separation of powers — remain the architecture of liberal democracy, even when liberal democracy is under stress.

To read these thinkers carefully is to be in conversation with the best minds of one of the most creative periods in human history. It is also to enter into the unfinished business of modernity, the work of building a world that lives up to its highest ideals — and the recognition that the work is unfinished precisely because the highest ideals were, in the first place, the highest ideals of slaveholders and colonial administrators, and that the question of whether they can be redeemed is the question that defines the present.