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An Engineer of Subversive Ideas

Denis Diderot loved the things that made others uneasy: ambiguity, change, doubt, and the pleasures of the flesh.


The early months of 1743 brought Denis Diderot’s career as a con artist and freeloader to a shameful end. His own father committed him as a prisoner to the Carmelite monastery in his hometown of Langres in eastern France, hoping to prevent his 29-year-old son from doing further harm to himself and others. Denis came from two respectable families: His father was a successful manufacturer of knives and surgical equipment, and his maternal uncle was a priest and administrator at Langres Cathedral. His parents and siblings cared deeply for him, and he received a first-class education. He could blame no one but himself.

Other Press, 320 pp., $28.95

He had been a rebel from the start. In school, Diderot peppered his Greek and Latin assignments with archaic grammar. When corrected by his teachers, he would explain his constructions to show that he knew the languages better than they did. One day he was expelled for fighting. Not wanting to miss that afternoon’s academic contests, he tried to sneak back in. The guard at the school gate stabbed him. As he approached adulthood, his family sent him to Paris to earn a degree in theology from the Sorbonne. He completed the five-year curriculum with ease but then neglected to fill out the paperwork for a place in the clergy and the comfortable pension that went with it. When his family asked him what he wanted to do with his life, he answered, “Nothing, nothing at all.”

The crisis came when he fell in love. Anne-Antoinette Champion (whom he called Toinette) was tall, assertive, pious, and destitute. Although she descended from aristocracy, her father lost a fortune in the Canadian fur trade and then suddenly died. She worked as a laundress in Paris, where she and Diderot met. Her mother was horrified that she wished to marry a man with no job and no prospects. Madame Champion didn’t know the worst of it. Diderot had recently scammed a Parisian church out of 2,000 livres (a skilled laborer might earn 400 livres per year). When he was caught, his father had to repay the sum. Returning to Langres to seek a reconciliation with his family, Diderot now told his father that he wished to marry Toinette even though she had no dowry. He then asked for an annual allowance of 2,500 livres for doing nothing, nothing at all. His father gathered some friends and forcibly incarcerated him in the nearby monastery. 

The story of how this reprobate became one of the most significant intellectual forces of the French Enlightenment is a central theme of Andrew S. Curran’s Diderot and the Art of Thinking Freely. Curran writes in the long shadow of Arthur M. Wilson’s Diderot (1972), one of the most thorough and eloquent biographies of any eighteenth-century figure. While Curran does not reveal any startling new facts about Diderot’s life or work, his fluent and spirited book surpasses its predecessor in finding a center of gravity in the Enlightenment’s most kaleidoscopic thinker. Diderot said of himself: “In the course of a single day I assumed a hundred different expressions, in accordance with the things that affected me. I was serene, sad, pensive, tender, violent, passionate, [and] enthusiastic.” Curran makes a persuasive case that Diderot’s mercurial temperament and writing style expressed not a lack of character or discipline but rather an unwavering intention to think (and feel) freely, to allow himself to consider every thesis and register every emotion without constraint.

First, however, he needed to free himself from the unmistakable constraint of his imprisonment. Diderot snuck out of the monastery at night and made the nearly 200-mile journey back to Paris through drenching spring storms. When he arrived, he found that his father had written to Toinette’s mother begging her to prevent the marriage and that Toinette had agreed to end the engagement. Penniless and heartbroken, Diderot became seriously ill. When Toinette heard the news, she tracked him down to a squalid apartment on the Île de la Cité, finding him emaciated and close to death. She and her mother nursed him back to health and, in doing so, their resistance apparently melted away. On October 5, 1743, Diderot turned 30, the age at which he could marry without his father’s permission. Denis and Toinette were wed at a midnight service at the church of Saint-Pierre in the heart of Paris. Recent events had chastened him. He would finally put his prodigious intellect to good use. 

Paris of the 1740s was flooded with clandestine books questioning the teachings of the church. Whether through contact with these works or through his own reflections, Diderot now repudiated his family’s pious Catholicism. The doctrine of original sin appeared absurd to him: How could a loving God condemn most of his people, including children, to eternal torment? (Diderot and Toinette had recently lost an infant.) Also, if God knows everything in advance, people are either saved or condemned from the beginning of time. The sacraments of the church are useless, and free will is an illusion. Diderot set down these ideas in his first substantial work, Philosophical Thoughts, which he published anonymously in 1746.

Instead of trying to prove the errors of religion through rigorous argumentation, he offered a series of 62 short essays giving voice to many views: deist, atheist, skeptic, and so on. “This is the genius of the book,” Curran observes. “Rather than pummeling us with an unrelenting and straightforward assault on the Catholic faith—something of which the author was more than capable—Diderot shares his own insecurities and hesitations about God’s existence.” As Montaigne had done two centuries before, Diderot developed a style of writing that conveyed the act of thinking rather than its results. The book was a hit, and as its popularity grew, so did curiosity about its author. A police inspector soon visited the Diderots at home and told Denis to be careful.

The real trouble came in 1749 with the publication of his Letter on the Blind. Superficially, the work concerns questions about vision that were much-discussed at the time, due in part to advancements in cataract surgery. For example, do a sighted mathematician and a blind mathematician have the same idea of a geometric figure? If not, why do they agree about geometric theorems? If they do, how is it possible given that they received their ideas through different senses—sight, and touch respectively? The controversial passages come at the end. Speaking in the voice of a dying, blind mathematician named Nicholas Saunderson, Diderot goes beyond epistemology to ask whether the universe needs a creator at all. If God must exist because everything has a cause outside itself, then what caused God? But if something can be self-caused, why not say it is the universe itself?

In a death-bed delirium, Saunderson hypothesizes that the universe is nothing but a churning, physical system, evolving through its own laws, creating and destroying whole galaxies over endless time. “How many lopsided, failed worlds are there that have been dissolved and are perhaps being remade and redissolved every minute in faraway spaces, beyond the reach of my hands and your eyes?” he asks. “Come with me to the edge of this universe, beyond the point where I can feel and you can see organized beings; wander across this new ocean with its irregular and turbulent movements and see if you can find in them any trace of that intelligent being whose wisdom you admire here.” 

Although Diderot presented these ideas as the visions of a dying man rather than as his own beliefs, they went too far for the government censors. Shortly after dawn on July 24, two officers entered Diderot’s apartment bearing a lettre de cachet. They searched his papers and placed him under arrest, taking him to solitary confinement in the Château de Vincennes east of Paris. A lettre de cachet was a fearsome instrument of royal prerogative, a direct order from the king that could not be appealed and carried no right to trial. Diderot might well have disappeared forever in the chateau’s dungeon.

Benefiting from the intervention of powerful friends, however, he was freed in November of that year, after more than 100 days of confinement. As part of his release deal, he signed a statement admitting that his writings were “debaucheries of the mind that escaped from me.” He promised, “on my honor (and I do have honor) that they will be the last.” For the most part, he kept his word. Although only in his mid-thirties at the time, he published little in the second half of his life. Instead, he set to work as editor of the most ambitious intellectual enterprise of the century.  

Before his arrest, Diderot and his friend Jean le Rond d’Alembert had hit on the idea of assembling a compendium of all human knowledge. Having secured printers to produce the work and subscribers to fund it, they now embarked on the “Systematic Dictionary of the Sciences, Arts, and Crafts” they called the Encyclopedia. The reign of dilettantes, obfuscating clerics, and secretive guilds was over, they insisted. This would be the work of experts and professionals, written for anyone who wished to know how the world operates.

“We distributed the appropriate piece to each,” Diderot boasted. “Mathematics to the mathematician; fortifications to the engineer; chemistry to the chemist… each person only what he understood.” While many of the articles were dry and technical, the enterprise had an ideological aim from the start. The point of the Encyclopedia, Diderot wrote, was to ensure that “the work of centuries past is not useless to the centuries which follow, that our descendants, by becoming more learned, may become more virtuous and happier, and that we do not die without having merited being part of the human race.”

Diderot soon took over as chief editor and for more than two decades, between 1750 and the publication of the final volume in 1772, guided and sustained a publishing project that staggers the mind. The Encyclopedia ran to 28 large volumes, containing more than 70,000 entries and 3,000 illustrations. Approximately 150 writers contributed to the work, producing more than 20 million words in total. They included Diderot, d’Alembert, Montesquieu, Jean-Jacques Rousseau, and Voltaire. The most prolific contributor was the Chevalier de Jaucourt, an aristocrat who had plenty of time on his hands and a stupendous work ethic. Between 1759 and 1765, with a team of researchers hired at his own expense, he wrote more than 17,000 articles, averaging about eight per day, equaling a quarter of the total word count.

To fulfill its ideological purpose, the Encyclopedia needed not only to explicate the most advanced knowledge of its time but also to encourage the attitudes that motivated these discoveries. It taught its readers to be bold, curious, iconoclastic, and independent. In doing so, it undercut much of the political culture and religious dogma of eighteenth-century France. The authorities stopped the project on two separate occasions, and Diderot lived in constant fear of arrest.

He “engineered the dissemination of subversive ideas,” as Curran puts it, through a series of brilliant and often hilarious stratagems. Most simply, he would sometimes commission an essay from a conservative thinker and then insert a passage of his own at the end that undercut the entire entry. In an article on the soul, for example, a Catholic apologist stated that the soul is the seat of reason, immaterial, immortal, and a separate substance from the body. After 17,000 words of tedious disquisition, the reader comes to a final note inserted by Diderot. Since the soul is separate from the body, it remains puzzling why a blow to the head affects one’s reason.

The Encyclopedia also makes pitiless use of irony. The article on “Noah’s Ark” offers a matter-of-fact discussion of what would be involved in building such a boat, populating it with animals, feeding them, and getting rid of their manure. Merely saying it shows its absurdity. By accepting dogmas at face value, the Encyclopedia mocks religious orthodoxy while appearing to honor it. Diderot’s most ingenious device, however, was his use of cross-references. The Encyclopedia often gives a straight-forward entry on a topic only then to include cross-references to unrelated articles implying things that could never be said. The entry on “Cannibals,” for instance, gives cross-references to “Communion” and “Eucharist.” 

For all this cleverness, the Encyclopedia did fulfill its goal of transmitting the cutting edge of science, technology, and engineering around Europe, the Atlantic world, and eventually the globe. Its success was due in no small part to its illustrations. A team of artists under Diderot’s direction spent years travelling everywhere from bakeries, hospitals, and iron foundries to military installations, pin factories, and shipyards. Their drawings were so accurate and detailed that a world-class technical school could be founded, or a nation’s economy could be reconfigured, by acquiring a copy of the Encyclopedia. The demand was insatiable. By the early 1780s, nearly 25,000 sets had been printed and sold.

While Diderot refrained from publishing anything controversial in his own name after his imprisonment, he continued to write. His most famous works today, aside from the Encyclopedia, were written to appear after his death. “Posterity,” he quipped, “is for the philosopher what the next world is for the man of religion.” The three most enduring of these, Rameau’s Nephew, D’Alembert’s Dream, and Jacques the Fatalist, subject Diderot’s philosophical views (primarily atheism and materialism) to the same skeptical interrogation that his earlier works employed against the teachings of the church. The books are astoundingly modern. Self-referential, occasionally self-contradictory, on the edge of spinning into nonsense, they give free rein to Diderot’s whirligig of a mind. The narrator of Rameau’s Nephew writes of himself: “I give in to my mind’s every fancy. I let it be my master and allow it to pursue the first idea that comes to it, good or mad.” Curran calls this pattern of thought Diderot’s “mental libertinage.”

As an original thinker, Diderot did not reach the level of Lucretius, Spinoza, or the other great philosophers whose ideas influenced his. Yet he excelled as a guide to philosophical concepts. In D’Alembert’s Dream, a fictionalized d’Alembert challenges the materialism of a fictionalized Diderot. If a soul is not necessary for consciousness, d’Alembert asks, why not say a stone can feel just as a human being does. “Why not?” Diderot replies: Take a stone statue, grind it up, mix it into the soil, grow a crop, and eat it. The matter is conscious when it is part of a person. Why not say it was conscious when it was part of a statue? Diderot’s vision of the universe as a vast expanse of combining and recombining matter led him to a host of original ideas. Perhaps, for example, hermaphroditism is the basic condition of animal life, from which male and female are deviants. Or, again, perhaps heterosexuality and homosexuality are not distinct categories but rather two extremes on a spectrum of desires. “Nothing that exists,” he wrote, “can be against nature or outside of nature.”

While Diderot and Toinette had risked all to be together, their marriage was not especially happy. Diderot became a workaholic and was also unfaithful to her. Toinette developed a fearsome temper and was physically violent. In 1755, Diderot met the woman who would be the significant influence on the later part of his life—a wealthy, brilliant, 38-year-old named Louise-Henriette Volland. He called her Sophie after the Greek word for wisdom. The 187 surviving letters that Diderot wrote to Sophie are treasures of eighteenth-century literature. One summer night in 1759, for example, he went to visit her but found that she was out or asleep and did not answer the door. In the pitch black, he took out a piece of paper and began to write.

I am writing without being able to see.... I am writing you that I love you.… I am unable to leave. The hope of seeing you for a moment is holding me back, and I continue to speak to you without knowing if I am actually forming letters. Wherever you see nothing [on this page], read that I love you.

Diderot’s gift of expressing his thoughts and feelings as they were happening to him achieves its most gripping effects in these letters.

As he reached old age, Diderot did not suffer the fears or regrets that his Christian friends expected he would. He was happy to imagine that the matter of which he was composed would be returned to the earth and reconfigured into something new. In one of his most touching letters, he imagines that he and Sophie would be buried next to each other so that their “ashes might press and mix together and unite.” He continues, “Maybe they haven’t lost all feeling or the memories of their earlier state…. I could touch you, feel you, love you, look for you, unite myself with you, and combine myself with you when we are no longer.” Curran says of this letter, “Diderot’s molecules scurry about in a quest for the carnal and intellectual joy that he had felt in an earlier life.”

Diderot and Toinette eventually reconciled, helping each other through serious illnesses late in life. By this time, they were a wealthy couple. The Encyclopedia had been an unprecedented success. Moreover, Catherine the Great, who admired the Encyclopedia and Diderot’s freewheeling intellect, offered him an extraordinary gift. She would buy his library for 15,000 livres on the condition that he keep the books as long as he lived. Moreover, she would pay him 1,000 livres per year to take care of his books and would pay the first 50 years in advance. (He would be 102 before the next installment was due.) The man who had almost starved to death as a young layabout and been twice imprisoned, once by his father and again by the king, spent his final days in a palatial Right Bank apartment supported by the Empress of Russia. 

In July of 1784, workmen arrived with a new bed for the elderly writer. As they labored to assemble it, Diderot told them, “My friends, you are giving yourself a lot of trouble for a piece of furniture that will only be used for four days.” He died the next afternoon. Diderot was different than most philosophers from Plato to the present. He loved the things that make others uneasy: ambiguity, change, doubt, idiosyncrasy, spontaneity, and the pleasures of the flesh. When his own body expired, suddenly and peacefully, he was with Toinette at their luncheon table, enjoying a second helping of dessert.