The most radical thinker of the eighteenth century, Denis Diderot (1713–1784), is not exactly a forgotten man, though he has been long overshadowed by his contemporaries Voltaire and Jean-Jacques Rousseau. After the French Revolution of 1789, the French right routinely blamed every ill of modern life on Voltaire and Rousseau. The expressions “It’s the fault of Voltaire” and “It’s the fault of Rousseau” became so familiar that Victor Hugo could satirize them in a ditty sung by the urchin Gavroche in Les Misérables (1862): “Joy is my character; ’tis the fault of Voltaire; Misery is my trousseau; ’tis the fault of Rousseau.” Voltaire and Rousseau were among the first to be buried in the French Pantheon of the nation’s heroes; Diderot has yet to be, despite a concerted campaign leading up to the three-hundredth anniversary of his birth in 2013.

Diderot was simultaneously too much a man of his time and too much ahead of his time. He devoted the best years of his life to organizing, editing, and writing many of the 74,000 articles of the Encyclopedia (1751–1772), a vast compendium of knowledge amounting to seventeen volumes of text and eleven volumes of plates, and laced with acerbic commentary that alarmed the authorities for attacking religion and subverting government. Known mainly to scholars today, at the time the project served as a thrilling treasury of Enlightenment ideas, if you knew where to find the nuggets hidden under the most unlikely headings. In the article “Nonetheless, However, Nevertheless, Notwithstanding,” for example, Diderot argued that even anti-Christian—i.e., atheist—writers could “nonetheless” be good parents, good friends, and good citizens. Since many articles were unsigned and the known contributors came from every corner of French life, no one could be sure what other ideas were Diderot’s.

As if that exhausting labor were not enough, Diderot anonymously contributed heaps of pages to another sprawling but influential work, this one on European colonialism. The History of the Two Indies appeared under the name of his friend the ex-Jesuit Guillaume Raynal. Diderot’s involvement in turning the second and third editions (1774 and 1780) into incendiary denunciations of European colonialism and the slave trade remained largely unknown before the second half of the twentieth century; the papers he left to his daughter were only inventoried in 1951, thanks to the work of Herbert Dieckmann, a German émigré professor then at Harvard, and scholars are still sorting out what came from Diderot’s pen.

As a man of his time, Diderot loved company and he loved Paris, the very place that Voltaire and Rousseau were always fleeing. His connection, unlike theirs, was not with public opinion but with the people he could talk to: his wife and daughter, his lovers, his countless friends, and, eventually, one ruling monarch in faraway Russia, Catherine the…