Diderot was born in Langres on October 5, 1713. The town, he would recall in the Encyclopédie article LANGRES, is fourteen leagues from Dijon, forty from Reims, sixty-three from Paris. His mother's family included several parish priests and a canon. His mother, the daughter of a tanner, and his father, a cutler, were very pious.

Denis was the eldest of seven children. The sister who followed him, Denise, remained a daughter, disfigured by skin cancer that destroyed her nose. Diderot loved her dearly. Another of Diderot's sisters, Angélique, entered the Ursulines against her parents' wishes, where she died insane at the age of 28. As for the family's youngest son, Didier, he became canon of Diderot's cathedral and never got on with his brother, whose religious views he did not tolerate.

In 1726, Denis received the tonsure from the bishop, the family hoping that he would thereby obtain the right to succeed his uncle Didier Vigneron as canon of the cathedral. But the uncle died prematurely, and the family project failed. By this time, the young Diderot was well acquainted with religious fervor. In 1728 or 1729, under unclear circumstances, he left Langres to pursue his studies in Paris, probably at the Jansenist Collège d'Harcourt, but perhaps also at the Jesuit Collège Louis-le-Grand, which Voltaire had attended before him. In September 1732 he was awarded a Master of Arts degree from the University of Paris.

His first works were translations from English: Temple Stanyan's Histoire de Grèce received the censor's approval, necessary for any publication, in May 1742. But he would also have worked on the edition of a mathematical work also in 1741. In 1748, his Mémoire sur différents sujets de mathématiques was published. During these years, Diderot assiduously frequented the theater:

Myself, as a young man, I swung between the Sorbonne and the Comédie. In winter, during the harshest season, I would recite roles by Molière and Corneille aloud in the lonely alleys of the Luxembourg. What was my plan? To be applauded? Perhaps. To live familiarly with the women of the theater, whom I found infinitely amiable and whom I knew to be very easy? Certainly. I don't know what I would have done to please the Gaussin, who was just starting out and who was beauty personified; the Dangeville, who had so many attractions on the stage." (Paradoxe sur le comédien, 1769-1773, Laffont, p. 1407.)

While living the Bohemian life of a student in the Latin Quarter, in 1741 Diderot met Anne-Toinette Champion, who traded in lace and linen with her mother. He fell in love with her and, in 1742, made the trip to Langres to ask her father for permission to marry her. A stormy scene ensued; the father refused and had Diderot locked up in a monastery, from where he fled and went into hiding in Paris. Diderot married in November 1743, his father no longer being able to disinherit him once he had reached the age of thirty. Relations between the couple would always be stormy, Mme Diderot being renowned for a difficult temperament, which was not improved by the hopeless insouciance of a husband who kept her apart from his world, like a shameful concubine.

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It was at this time (August 1742) that Diderot befriended Rousseau, who had just arrived in Paris with a new system of musical notation of which he was the inventor.

Diderot's main source of income at this time was his translations from English. He paraphrased (rather than translated) Milord Shaftesbury's An Inquiry concerning Virtue and Merit, which he published in Amsterdam in 1745 under the title Principles of Moral Philosophy or Essai de M. S... sur le mérite et la vertu. Avec réflexions. Published outside France and without an explicit author's name, the book says enough about its subversive content: Shaftesbury's morality is founded in nature, independently of religion and Scripture. Yet the review in the Journal de Trévoux, a journal of Jesuit obedience, at the head of the February 1746 issue, is favorable. After Shaftesbury, Diderot translates, this time without commentary, Robert James's Dictionnaire Universel de Médecine, published in London between 1743 and 1745. The French version was published by Briasson from 1746 to 1748. Also from this period date the notes for a French translation of Pope's Essai sur l'homme.

But Diderot's first original work was his Pensées philosophiques, bought by Durand (he would become, along with Briasson, one of the booksellers associated with the publication of the Encyclopédie) and printed clandestinely by a man named l'Épine in 1746. The book was condemned by the Paris Parliament in July to "be lacerated and burned [...] by the executor of the High Justice as scandalous, contrary to religion and good morals." The book, according to the Parliament's ruling,

"presents to restless and reckless minds the venom of the most criminal and absurd opinions of which the depravity of human reason is capable; and by an affected uncertainty, places all religions almost in the same rank, to end by recognizing none."

The consequence of this ruling was the opposite of what it was intended to achieve: Diderot thus made a name for himself, and his book went through ten editions in the century, not to mention a German translation and five works which, under cover of refuting it, quoted it in extenso!

Diderot, living in virtual hiding in Paris with his wife and a son, didn't stop there, however. He wrote De la suffisance de la religion naturelle (1746), which was not published until 1770, and a great allegory, La Promenade du sceptique (1747), which the police prevented him from publishing, either by extorting the promise or confiscating his manuscript. The book was not published until 1830.

Then began the adventure of the Encyclopédie.