In 1749 the police arrested a struggling Paris writer and locked him in the fortress of Vincennes for publishing a little book that questioned God's existence through a discussion of blind people. He was released after about three months, partly because his publishers pleaded that they needed him. He was the editor of a giant reference work they had invested in. The writer was Denis Diderot, and the work was the Encyclopédie, which would take him more than twenty years and become the great collective project of the French Enlightenment.

From translation to revolution in print

The project began modestly. In 1745 the Paris publisher André Le Breton planned a French translation of Ephraim Chambers's English Cyclopaedia. By 1747 the editors were Diderot and the mathematician Jean le Rond d'Alembert, and the plan had grown into something new. Its full title promised a Reasoned Dictionary of the Sciences, Arts and Crafts. Between 1751 and 1772 it grew to 17 volumes of text and 11 volumes of engraved plates, with roughly 72,000 articles written by well over a hundred contributors.

A new map of knowledge

D'Alembert's Preliminary Discourse (1751) laid out a tree of knowledge adapted from Francis Bacon. Knowledge branched from three faculties of the mind: memory (history), reason (philosophy and science) and imagination (the arts). Theology appeared as just one branch under reason, alongside other subjects. Readers noticed. Simply rearranging the map of knowledge implied that religion should be examined like anything else.

The dignity of work

One of the most original features of the Encyclopédie was its respect for the mechanical arts. Diderot visited workshops, questioned craftsmen and commissioned detailed engravings of everything from pin making and glassblowing to printing and weaving. Learned books had rarely taken manual work so seriously. The plates showed that the knowledge in a weaver's hands was real knowledge, worth recording and improving, and they remain one of the best visual records of eighteenth-century work.

Censorship and survival

The Encyclopédie lived under constant threat. In 1752, after a scandal involving a contributor's theology thesis, the royal council suspended it. In 1759 its official permission to publish was revoked and the pope placed it on the Index of Forbidden Books. D'Alembert, worn down, withdrew from the editing. Diderot carried on, and the remaining text volumes were printed quietly and released together in 1765.

Surprisingly, part of its protection came from inside the government. Malesherbes, the official in charge of the book trade, sympathized with the project. When a police raid was coming, he is said to have warned Diderot and offered to hide the manuscripts in his own house. The episode shows how Enlightenment ideas had friends within the very institutions they criticized.

Diderot also suffered a betrayal from his own side. In 1764 he discovered that Le Breton, afraid of prosecution, had secretly cut passages from the final volumes after Diderot had approved the proofs. Diderot was furious but kept working.

The contributors

The writers ranged from Voltaire and Rousseau, who wrote on music and political economy, to economists, doctors and engineers. The most tireless was the Chevalier Louis de Jaucourt, who wrote about a quarter of all the articles, often working at his own expense. The result was less a unified doctrine than a meeting place, a printed version of the Republic of Letters.

The Encyclopédie was also a business. Its first edition sold about four thousand copies, an enormous run for such an expensive work, and cheaper reprints and pirated editions printed in Switzerland and Italy carried it to many thousands more readers across Europe. The historian Robert Darnton has shown how these later editions turned it from a luxury for aristocrats into a work owned by lawyers, officials, doctors and clergy in provincial towns. Diderot himself earned relatively little from it, and he sold his library to pay for his daughter's dowry, a story that returns in a later level.

Worked example: reading between the cross-references

Writers under censorship learn to say things indirectly. The Encyclopédie used its cross-references, the "see also" links at the end of articles, as a hidden channel. Diderot explained that they could be used to connect an orthodox article to one that quietly undermined it.

• The surface: the article on cannibals, "Anthropophages," describes people said to eat human flesh. Nothing in it would trouble a censor.
• The link: it ends by sending the reader to the articles on the altar, communion and the Eucharist.
• The implication: the reader is invited to compare cannibalism with the Catholic doctrine that the bread of communion becomes the body of Christ.
• The defense: each article, read alone, could be presented as harmless.

The lesson cuts two ways. Clever structure can carry ideas past censors, but the same technique can be used to plant suggestions without stating them openly enough to be challenged. A careful reader notices where a text is steering them, and asks whether the hint would survive being said out loud.