In December 1783 a Berlin clergyman named Johann Friedrich Zöllner added a grumpy footnote to a magazine article. Everyone was talking about enlightenment, he complained, but what is it? Within a year two of Germany's leading thinkers had answered him in the same journal, the Berlinische Monatsschrift. The more famous answer, by the philosopher Immanuel Kant, appeared in December 1784 and opened with a line still quoted today: enlightenment is humanity's emergence from its self-incurred immaturity. This course is about the people who tried to live up to that line, and about what they got right and wrong.

A question in a footnote

Kant meant something precise by immaturity. It is not a lack of intelligence but a habit of letting others think for us: the book that serves as our understanding, the pastor who serves as our conscience, the doctor who decides our diet. It is self-incurred because the cause is not stupidity but a lack of courage. His motto was borrowed from the Roman poet Horace: Sapere aude, dare to know, or have the courage to use your own understanding. Kant added that the most important freedom for enlightenment is the freedom to make public use of one's reason, to argue openly before the reading public.

The other answer came from Moses Mendelssohn, a Jewish philosopher who had arrived in Berlin as a poor teenager and taught himself German, Latin and philosophy. That a self-taught Jewish scholar and a Prussian professor could both be asked to define the age says something about it already.

When and where

Historians usually place the Enlightenment between the 1680s and about 1800. Its opening works include Isaac Newton's Principia (1687), John Locke's Essay Concerning Human Understanding (1689) and Pierre Bayle's sprawling, skeptical Historical and Critical Dictionary (1697). Its close is often tied to the French Revolution, which put many Enlightenment ideas to a harsh practical test.

Paris is the city most associated with the movement, and the French word philosophe came to mean a writer who used reason to criticize society, not just a philosopher in the academic sense. But there were lively centers in Edinburgh, London, Amsterdam, Berlin, Milan, Naples, and Philadelphia, each with its own concerns. Some historians now prefer to speak of Enlightenments in the plural.

The Republic of Letters

Enlightenment thinkers imagined themselves as citizens of a Republic of Letters, an international community that existed mostly on paper. Its members wrote to each other across borders and languages, reviewed each other's books, and argued in journals. Voltaire alone left behind many thousands of letters. The republic had no territory and no king, and in principle its only authority was the quality of an argument. In practice it was dominated by educated, mostly male, mostly well-off Europeans, a limit we will return to in the final level.

Shared commitments

The philosophes disagreed about almost everything, including God, government and human nature. Still, most of them shared a family of commitments:

• Reason and experience over tradition and revelation as the test of a claim.
• Toleration, especially in religion, after more than a century of religious wars and persecution.
• Criticism of established authority, from churches to courts to censors.
• Improvement, the hope that knowledge could make human life healthier, freer and less cruel.
• Humanity, a growing sense that all people share a common nature that deserves respect.

Moderate and radical

The historian Jonathan Israel distinguishes a moderate Enlightenment, which hoped to reconcile reason with religion and existing institutions, from a radical Enlightenment inspired partly by the Dutch philosopher Baruch Spinoza, which rejected revelation altogether and pushed toward democracy and equality. Not every scholar accepts the division, but it is a useful reminder that the Enlightenment was an argument, not a single program.

Why it happened when it did

Several forces came together in the late seventeenth century. Europe was exhausted by religious wars, including the Thirty Years' War, which had devastated central Europe until 1648, and many people longed for a way to settle disputes that did not depend on whose church was right. Trade and empire brought news of other civilizations, such as China, whose ethical traditions impressed writers like Voltaire and showed that morality did not require Christianity. Printing had become cheap enough for books and periodicals to reach a growing middle class. And the success of the new natural science suggested that careful reasoning could achieve what centuries of authority had not. Enlightenment was, in part, a response to these pressures.

Worked example: reading Kant's answer

Kant claimed that his own time was not yet an enlightened age but an age of enlightenment. What did he mean, and how could you apply the distinction?

• Enlightened age: one in which people already use their own reason freely, without guidance from authority, on all important matters. Kant said Prussia was nowhere near that.
• Age of enlightenment: one in which the obstacles are being cleared, so that people are increasingly free to think and argue in public.
• The test: ask not whether people hold correct opinions, but whether they are free and willing to reason for themselves and to argue openly.
• Applying it today: a society full of information can still be immature in Kant's sense if people simply hand their judgment over to a favorite source.

Kant's distinction turns the Enlightenment from a finished period into an ongoing task. That is the spirit in which this course approaches it.