Enlightenment thinkers and their ideas

What the Enlightenment was, and when

Historians usually start the Enlightenment in the 1680s, after Newton's Principia (1687) and John Locke's Two Treatises of Government and Letter Concerning Toleration (both 1689), and end it with the French Revolution in 1789 or soon after. It was not a single school. There was a French Enlightenment of satirists and encyclopedists, a Scottish Enlightenment of philosophers and economists in Edinburgh and Glasgow, a German Aufklärung, and an American version that ended up writing constitutions.

In 1784 the philosopher Immanuel Kant answered the question “What is Enlightenment?” with a motto from the Roman poet Horace: Sapere aude, “dare to know”, or have the courage to use your own understanding without another's guidance. Most Enlightenment writers shared a handful of ideas:

• Reason and evidence, not tradition or church authority, should decide what is true. Newton's success in physics was the model.
• People have natural rights (Locke named life, liberty and property) and government rests on the consent of the governed.
• Power should be divided so no one can abuse it: Montesquieu's The Spirit of the Laws (1748) set out the separation of legislative, executive and judicial powers.
• Religious toleration, and sharp criticism of persecution and clerical power.
• Laws and punishments should be useful and humane: Cesare Beccaria's On Crimes and Punishments (1764) attacked torture and the death penalty.
• Knowledge should be shared: Diderot and d'Alembert's Encyclopédie (1751 to 1772) ran to 28 volumes of text and plates.

These ideas spread through printed books and newspapers, coffee houses, Paris salons often run by women, learned societies and a huge web of letters that writers called the Republic of Letters.

The inspiration: Newton

Isaac Newton (1643 to 1727) was not an Enlightenment campaigner, and he was a devout if unorthodox Christian. But his Principia showed that a few mathematical laws could explain the motion of everything from apples to planets, and his admirers concluded that society might be explained and improved the same way. Voltaire popularised Newton's physics in France in the 1730s, helped by Émilie du Châtelet, whose French translation of the Principia is still in use. See key figures of the Scientific Revolution.

The European thinkers: Voltaire, Adam Smith and Catherine the Great

Voltaire (1694 to 1778) was the pen name of François-Marie Arouet, a Paris playwright twice locked in the Bastille who spent 1726 to 1728 in England. His Letters Concerning the English Nation (1733 in English, 1734 in French) praised English religious variety, Locke, Newton and smallpox inoculation; the Paris authorities had the French edition burned. His satire Candide (1759) mocked the idea that all is for the best after the Lisbon earthquake of 1755, and includes an enslaved man in Surinam mutilated to produce Europe's sugar. In 1762 the Protestant merchant Jean Calas of Toulouse was tortured and executed on a false charge of killing his son; Voltaire's campaign, including the Treatise on Tolerance (1763), won a posthumous reversal of the verdict in 1765. Two cautions: he also wrote hostile, stereotyping passages about Jews, and the famous line “I disapprove of what you say, but I will defend to the death your right to say it” was written by his biographer Evelyn Beatrice Hall in 1906, not by him.

Adam Smith (1723 to 1790) was a Scottish professor of moral philosophy at Glasgow and a close friend of the philosopher David Hume. His first book, The Theory of Moral Sentiments (1759), is about sympathy and conscience, not money. The Wealth of Nations (1776) opens with a pin factory where dividing the work into about eighteen operations lets ten workers make some 48,000 pins a day, and argues that free exchange and competition usually serve the public better than monopolies and state favours. He used the phrase “invisible hand” only once in that book, warned that merchants of the same trade rarely meet without conspiring against the public, called government by the East India Company one of the worst of governments, and argued that slave labour was in the end the most expensive of all.

Catherine the Great (1729 to 1796) was a German princess who took the Russian throne in a coup against her husband, Peter III, in 1762. She wrote to Voltaire for fifteen years, bought Diderot's library in 1765 and let him keep it while paying him as its librarian, and in 1767 issued the Nakaz, an instruction for a new law code that borrowed heavily from Montesquieu and Beccaria. In 1768 she had herself inoculated against smallpox to set an example. Yet she strengthened noble power over the serfs, crushed Pugachev's rebellion (1773 to 1775), took part in all three partitions of Poland, and in 1790 had the writer Alexander Radishchev sentenced to death, commuted to Siberian exile, for a book exposing serfdom. Historians call rulers like her “enlightened absolutists”, and argue over how much the first word means.

The American founders: Franklin, the Adamses and Hamilton

Benjamin Franklin (1706 to 1790) was a Philadelphia printer who founded a subscription library (1731) and the American Philosophical Society (1743), and whose Experiments and Observations on Electricity (1751) made him famous in Europe. He sat on the committee that drafted the Declaration of Independence, won the French alliance of 1778 and signed the Constitution in 1787, urging the delegates to doubt their own infallibility. He owned enslaved people for much of his life; in 1790, as president of the Pennsylvania Abolition Society, he signed a petition asking Congress to act against slavery and the slave trade.

John Adams (1735 to 1826) was a Boston lawyer who defended the British soldiers after the Boston Massacre in 1770. His pamphlet Thoughts on Government (1776) recommended a two-chamber legislature and an independent executive and judiciary, and as main author of the Massachusetts Constitution of 1780 he wrote in the phrase “a government of laws and not of men”. As second President (1797 to 1801) he also signed the Alien and Sedition Acts of 1798, which punished criticism of the government.

Abigail Adams (1744 to 1818) ran the family farm and finances while John was away and wrote him more than a thousand letters. On 31 March 1776 she pressed the Enlightenment's own logic further than the men did:

John's reply of 14 April said he could not but laugh. Two years earlier, in September 1774, she had called it iniquitous to fight for liberty while holding others in slavery. See Remember the Ladies.

Alexander Hamilton (1755 or 1757 to 1804) was born in the Caribbean, served on George Washington's staff and wrote 51 of the 85 Federalist essays in 1787 and 1788. The first of them states the Enlightenment wager plainly: whether people can set up good government “from reflection and choice”, or must always depend on “accident and force”. As first Secretary of the Treasury he built the federal debt, tax and banking system. Compare him with Adams in Hamilton vs Adams.

Enlightenment timeline

Big themes and debates about the Enlightenment

• Rights for whom? The same writers who praised natural rights often left out women, enslaved people and colonised peoples, and some, such as Hume and Kant, wrote racist passages. Others, such as Condorcet, and Black writers such as Olaudah Equiano, used Enlightenment arguments against slavery. Students should cite both.
• Enlightened despots. Catherine the Great, Frederick II of Prussia and Joseph II of Austria borrowed Enlightenment language while keeping absolute power. Was it reform or public relations? The Nakaz and the treatment of Radishchev make a good test.
• Religion. Most Enlightenment figures were not atheists. Voltaire and Franklin were deists who believed in a creator; their target was persecution and clerical power.
• Did ideas cause revolutions? Historians disagree on how far books caused the American and French Revolutions, compared with taxes, debt, war and bread prices. The Haitian Revolution forced the question of whether the rights of man applied to everyone.
• One Enlightenment or many? Some historians, such as Jonathan Israel, stress a radical Enlightenment of democrats and freethinkers; others stress moderate reformers working inside churches and monarchies.

Primary sources to read

• John Locke, Second Treatise of Government (1689), chapters 2 and 9: the state of nature and why people form governments.
• Montesquieu, The Spirit of the Laws (1748), Book 11, chapter 6: the separation of powers, using England as his example.
• Voltaire, Candide (1759), chapters 1 to 6 and 19, and the Treatise on Tolerance (1763), chapter 1 on the Calas case.
• Adam Smith, The Wealth of Nations (1776), Book 1, chapter 1: the pin factory.
• Kant, An Answer to the Question: What is Enlightenment? (1784): only a few pages.
• Abigail Adams to John Adams, 31 March 1776, and his reply of 14 April 1776, in the Adams Family Papers.
• The Federalist No. 1 (Hamilton) and No. 51 (Madison).
• Mary Wollstonecraft, A Vindication of the Rights of Woman (1792), introduction.
• Catherine the Great, the Nakaz (1767), the opening articles.

How to study the Enlightenment: projects

• Run a salon. Each student takes one thinker, prepares a two-minute summary of one idea with a quotation from the source, and the host keeps the conversation moving, as Paris hostesses such as Madame Geoffrin did.
• Idea tracker. Take three ideas (natural rights, separation of powers, toleration) and trace each from its Enlightenment source into the Declaration of Independence, the US Constitution or the 1789 Declaration of the Rights of Man.
• Test the enlightened despot. Put Catherine's Nakaz next to her 1785 Charter to the Nobility and decide which tells you more about her rule.
• The letter project. Write a reply to Abigail Adams's 31 March 1776 letter from a different person in the household, then compare it with John's real reply.
• Interview a thinker. Use our guide to writing interview questions, then practise by voice with Voltaire on Yuga or Benjamin Franklin. These are AI recreations built from their writings and the record, not the people themselves, and they can be wrong, so cite the original texts in anything you hand in.