The Enlightenment is under very bad weather right now. The French eighteenth-century movement that once was seen to have bathed Europe in the light of reason—fighting for science against superstition, and for liberty against bondage—has become the villain of many a postmodern seminar and of even more revisionist histories, from left and right alike. The Enlightenment’s supposed faith in reason—its desire to be sure that every “passion’ll / soon be rational,” to adapt the enlightened Ira Gershwin—is held responsible for racism, colonialism, and most of the other really bad isms. Enlightenment order is now understood as overlord violence pursued through other means. Its true symbol is not some peaceful Temple of Reason but the Panopticon—the all-surveying, single-eye system of Jeremy Bentham’s ideal prison. Where pre-Enlightenment Europe was sporadically cruel, post-Enlightenment Europe was systematically inhumane; where the pre-Enlightenment was haphazardly prejudiced, the Enlightenment was systematically racist, creating a “scientific” hierarchy of humanity that justified imperialism. “Reason” became another name for bourgeois oppression, the triumph of science merely an excuse for more orderly forms of social subjugation.

Well, all views produce counter-views, but—and this is one of the lessons of the Enlightenment itself—they tend to come less often from within the era’s Academy of Orthodoxy than from traditions blooming outside it. So, these days, the anti-Enlightenment view is countered most potently by a set of parallel popular enthusiasms. Outside academia, the Enlightenment is not just in good odor but practically Hermès-perfumed. Voltaire has been the subject of (by my count) five popular and mostly positive biographies in the past decade alone, and now the brightest Enlightener of them all, Denis Diderot, is being newly enshrined in two fine books written by American scholars for a general audience: Andrew S. Curran’s “Diderot and the Art of Thinking Freely” (Other Press) and Robert Zaretsky’s “Catherine & Diderot” (Harvard), an account of Diderot’s legendary collusion with a Russian autocrat.

Diderot is known to the casual reader chiefly as an editor of the Encyclopédie—it had no other name, for there was no other encyclopédie. Since the Encyclopédie was a massive compendium of knowledge of all kinds, organizing the entirety of human thought, Diderot persists vaguely in memory as a type of Enlightenment superman, the big bore with a big book. Yet in these two new works of biography he turns out to be not a severe rationalist, overseeing a totalitarianism of thought, but an inspired and lovable amateur, with an opinion on every subject and an appetite for every occasion.

He was and remains, as Zaretsky says simply, a mensch. He is also a very French mensch. He is a touchingly perfect representative—far more than the prickly Voltaire—of a certain French intellectual kind not entirely vanished: ambitious, ironic, obsessed with sex to a hair-raising degree (he wrote a whole novella devoted to the secret testimony of women’s genitalia), while gentle and loving in his many and varied amorous connections; possessed of a taste for sonorous moralizing abstraction on the page and an easy temporizing feel for worldly realism in life; and ferociously aggressive in literary assault while insanely thin-skinned in reaction, littering long stretches of skillful social equivocation with short bursts of astonishing courage.

It has been said that there were two Enlightenments, one high and one low. The high Enlightenment was the Enlightenment that produced the weighty works and domineering ideas; the low, or popular, Enlightenment was—in ways that scholars as unlike as Jürgen Habermas and Robert Darnton have been illuminating for the past half century—the Enlightenment of the cafés and conversation, or, at times, of pamphleteering and pornography.

Until the moment, in the late seventeen-forties, when he was asked to undertake the Encyclopédie, Denis Diderot was mainly a figure of the low Enlightenment, and might have seemed a quite improbable encyclopedist. The ne’er-do-well son of a wealthy provincial bourgeois family, he ducked out of an apprenticeship in law and became a figure of the cafés, known for his conversation and social amiability. His friendship with Jean-Jacques Rousseau, which lasted for nearly twenty years—longer than almost anyone else sustained a friendship with the ornery and paranoid Swiss philosophe—began when they met drinking coffee and playing chess in the Café de la Régence, one of the cafés clustered around the Palais Royal, in Paris, where the real reservoir of Enlightenment social capital was produced. Diderot has such an engaging aura in his writing that an idealized Fragonard portrait of a reader at work—open collar, wigless, bright-eyed and wry—was, until 2012, falsely identified as Diderot. (He isn’t nearly so handsome in any of the surviving frontispieces to his work.) It was the way Diderot ought to have looked, even if he didn’t.

From an early age, he loved women and women loved him back. (His marriage to, of all people, an oddly wellborn working laundress named Toinette was not a success; she would have street brawls with his mistresses.) He had what we call charm, the ability to present intelligence as though it were identical with amiability: he knew that we are sooner seduced by someone who is smart enough to enlist our sympathy than by someone who tries to enlist our sympathy by being smart. Almost alone among his peers, he was presciently aware that chattering could be a way of mattering. “What we write influences only a certain class of citizen,” he once wrote about his circle of confrères, “while our conversation influences everyone.” He understood that civil society, radiating out from the small circles of the cafés to a larger civilization, could change public opinion, noting “the effect of a small number of men who speak after having thought,” and whose “reasoned truths and errors spread from person to person until they reach the confines of the city, where they become established as articles of faith.” Minds made talk; talk made minds.

One couldn’t just drink coffee and talk and still make a living, though—especially after Diderot was disinherited for his bohemianism by his bourgeois dad. He became a miscellaneous essayist and translator, scuffling to make a living by writing political pamphlets, philosophical dialogues, and pornographic books—all the while carrying on vigorous romantic liaisons with a variety of partners, from the local washerwoman to aristocratic readers. His fortunes were boosted by his first popular hit, the 1748 novel “Les Bijoux Indiscrets,” or “The Indiscreet Jewels,” which was a sort of “Dangerous Liaisons” of lingerie. Though “Les Bijoux” acquired a reputation as a “ribald classic,” it has a more than respectable literary pedigree; a regular theme of the French Enlightenment was that the way we love and the way we learn, the forms of sensual desire and the forms of scientific description, might be intimately connected.

Still, “Les Bijoux Indiscrets” must be among the strangest books of philosophical pornography ever published, even in that highly competitive French Enlightenment division. Its story tells of a sultan, evidently a correlate for Louis XV, who acquires a magic ring that empowers, or compels (the sexual politics here are tricky), vaginas—those bijoux, or jewels—to tell their true histories from within women’s underwear. (A contemporary English translation calls them, perhaps more in Diderot’s spirit, “toys.”) One after another narrates a tale, typically of unrepentant infidelity to its official male “owner.” These revelations, treated more as genial truths than as adulterous shocks, give way from time to time to broader speculations. (“The soul remains in the feet to the age of two or three years; at four it inhabits the legs; it gets up to the knees and thighs at fifteen.”) The climax of the book, a marriage of pornography and the philosophy of science which, in modern terms, could have been written only by Karl Popper in collaboration with Terry Southern, occurs when the sultan has a dream in which he sees Plato and his followers, who are blowing bubbles in a temple of Hypothesis (i.e., mired in idle philosophical speculation). Suddenly, an expanding phallic figure appears: