In the summer of 1793 the Marquis de Condorcet, mathematician, philosophe, champion of women's rights and opponent of slavery, was declared an outlaw by the revolutionary government he had helped to create. He hid for months in a Paris boarding house. There, knowing he might be caught at any moment, he wrote a sketch of the progress of the human mind, predicting a future with less inequality between nations, between classes and between the sexes. In March 1794 he was arrested and was found dead in his cell two days later. His book was published the following year. Condorcet's story holds the Enlightenment's highest hopes and its most painful ironies in a single life.

Rousseau's dissent

The sharpest early critic of the Enlightenment came from inside it. Jean-Jacques Rousseau contributed to the Encyclopédie, but his prize essay of 1750 argued that the arts and sciences had made people more polished and less virtuous. He thought civilization bred vanity, inequality and dependence on others' opinions. He quarreled bitterly with Voltaire, Diderot and Hume. Rousseau did not reject reason, but he insisted that feeling, conscience and simple living mattered as much, and his ideas fed the Romantic movement that followed.

The hardest contradiction

Enlightenment writers spoke of universal humanity while the Atlantic slave trade reached its peak. Some of them made the contradiction worse. Hume added a footnote to an essay claiming that Africans were naturally inferior to Europeans. Kant wrote racial hierarchies into his lectures on geography and anthropology. Thomas Jefferson wrote that all men are created equal and enslaved more than six hundred people over his lifetime.

Yet Enlightenment arguments also became the most powerful weapons against slavery. Montesquieu mocked the excuses for it with bitter irony. The History of the Two Indies, published under the name of the Abbé Raynal with passages by Diderot, condemned colonial slavery and predicted a revolt of the enslaved. Condorcet wrote against slavery in 1781 and joined the Society of the Friends of the Blacks, founded in Paris in 1788. Enslaved and formerly enslaved writers, such as Olaudah Equiano, whose autobiography appeared in 1789, used the same language of natural rights to demand freedom.

Counter-Enlightenment and later critics

The German writers Johann Georg Hamann and Johann Gottfried Herder objected that the philosophes judged all peoples by a single standard of reason. Herder argued that each culture has its own value and must be understood on its own terms. The historian Isaiah Berlin later grouped such thinkers as the Counter-Enlightenment. In the twentieth century, Max Horkheimer and Theodor Adorno argued in Dialectic of Enlightenment that the drive to master nature through reason could turn into domination of people. Other critics have asked how far its universal language served European empire.

The French Revolution complicated the Enlightenment's reputation further. Revolutionaries quoted Voltaire and Rousseau, and the Declaration of the Rights of Man and of the Citizen (1789) echoed Enlightenment language. Yet the Terror of 1793 and 1794 sent thousands to the guillotine in the name of reason and virtue, and it claimed some of the last philosophes, including Condorcet and the chemist Antoine Lavoisier. Critics on the right, such as Edmund Burke, blamed abstract reasoning for the violence. Defenders replied that the Terror betrayed Enlightenment values of toleration and due process rather than fulfilling them. Historians still debate how direct the connection was.

Why it still matters

Much of what we take for granted traces to these debates: religious toleration, freedom of the press, the critique of torture, public education, the scientific study of society, and the idea that institutions should be judged by how they treat human beings. In 1984, exactly two hundred years after Kant's essay, the French philosopher Michel Foucault, himself a critic of many Enlightenment claims, suggested that its real legacy is not a set of doctrines but an attitude: a permanent critique of our own time.

Worked example: defending the Enlightenment honestly

A friend says the Enlightenment was hypocritical, because its heroes preached equality and excused slavery. How do you respond without either denying the charge or abandoning the ideals?

• Grant the facts: Hume's footnote, Kant's racial theories and Jefferson's slaveholding are real, and pretending otherwise is not honest.
• Separate the principle from its holders: the claim that all humans share a nature deserving respect was right, even when the people who stated it failed to live by it.
• Follow the argument forward: abolitionists, feminists and anticolonial writers used Enlightenment principles to expose those failures.
• Apply the method to ourselves: the habit of criticizing received ideas is the best tool for finding our own blind spots.

This is where the Enlightenment meets Universal Humanism. Universal Humanism takes the Enlightenment's best commitments, reason, evidence, toleration and the equal dignity of every person, and extends them to everyone the eighteenth century left out. Kant's question remains open. Each generation has to dare to know for itself, and to make sure that the light reaches every human being.