Two decades ago, while studying in Paris, I stumbled across the wonderful bookstore of the Libre Pensée (“Free Thought”) association. Tucked away in the narrow streets of the Fifth Arrondissement, near where Balzac set Le Père Goriot, it seemed a relic of the past: shabby, dim and cluttered, the gentle odor of old book leather hinting at hidden treasures on the shelves. And indeed, most of the books appeared, delightfully, to have come out of an alternate timeline in which the age of Voltaire had never ended. Thick volumes denounced the hidden conspiracies of the Jesuits and chronicled the crimes of the Inquisition. Pamphlets offered concise defenses of Deism. And a rickety postcard stand displayed, alongside the usual images of the Pantheon and the Seine, engravings of the execution of Jean Calas, an eighteenth-century victim of religious bigotry posthumously exonerated through the efforts of Voltaire. One half expected to see the philosophe himself browsing quietly at the next shelf.

Back then, in the mid-1980s, La Libre Pensée seemed like nothing more than a pleasant curiosity. After all, the great battles of the Enlightenment had burned out long before. Religious intolerance and fanaticism were no longer matters of major concern. Indeed, for many of my French fellow students, the great enemy was the Enlightenment itself. Every week they would cram into a crowded lecture hall at the Collège de France to hear Michel Foucault, then in the last year of his life, explain how the eighteenth century saw the imprisoning of the Western world in a straitjacket of mental discipline. They struggled to grasp the quicksilver sentences in which Jacques Derrida deconstructed the criteria of rationality and truth that eighteenth-century philosophy had taken as axiomatic. They spoke derisively of an Enlightenment that had culminated not in modern democracy but in Auschwitz.

Today, things look rather different. Pace Foucault, enlightened psychiatrists and prison reformers do not seem particularly dangerous compared with suicide bombers and book burners. In the twenty-first century the Enlightenment appears anything but the triumphant imperial “project” denounced by vulgar postmodernists. Its heritage is fragile and endangered. Admittedly, its works remain in the “canon”–but perhaps only because they go largely unread in certain quarters. I sometimes wonder what would happen if, for instance, a public university system asked all entering students to read Voltaire’s Philosophical Dictionary, with its deep, deliberate offensiveness toward Christianity. What if a major studio attempted to film his Mahomet, a play far more systematically disparaging of the Prophet than Rushdie’s Satanic Verses?

If nothing else, though, our own century’s retreat from the Enlightenment should help us appreciate that this great movement of ideas has been a fragile thing all along, and never more than in its supposed eighteenth-century heyday. Its swift apparent triumph in the American and French revolutions makes it easy to forget that its greatest authors constantly ran afoul of the authorities and took great personal risks throughout their careers. Both Voltaire and Jean-Jacques Rousseau, as these two excellent new biographies point out, had books officially shredded and burned in Paris–even if, in Voltaire’s case, the responsible official surreptitiously substituted another volume at the last minute, so as to take the offending one home for himself. Roger Pearson reminds us that Voltaire served several stints in prison, lived much of his life in exile from his native France and had to have the sheets of his first great work, the Philosophical Letters, smuggled into Paris in a furniture cart. Even his most famous patron, Prussia’s Frederick the Great, once remarked of him: “I shall have need of him for another year at most, no longer. One squeezes the orange and throws away the peel.” As for Rousseau, Leo Damrosch recalls that the threat of prosecution kept him constantly on the move among France, Switzerland and Britain. “I never get a virtuous or useful idea,” Rousseau wrote to his patron Malesherbes in 1765, “without seeing the gallows or the scaffold before me.”

The remark may seem overly dramatic, even paranoid–and Rousseau very likely did suffer from clinical paranoia in his last years. But consider that in 1766, a 21-year-old nobleman named Jean-François de La Barre was publicly tortured, decapitated and burned in the northern French town of Abbeville for the “crime” of impiety–with the charge proved by nothing more than the fact that he owned a copy of Voltaire’s Philosophical Dictionary. The executioner threw the book onto the funeral pyre next to his victim’s head.

How, then, did the Enlightenment manage to survive at all, let alone flourish? To answer this question, historians generally try to locate figures like Voltaire and Rousseau within large-scale cultural shifts. They talk of the general weakening of religious orthodoxy, despite cases like de La Barre’s, and the declining ability of the Christian churches to impose beliefs on the supposed faithful. They invoke the vertiginous expansion of the book market, and the even more vertiginous rise of institutions–lending libraries, coffeehouses, newspapers, reading circles–that brought Enlightenment ideas to the attention of a new reading public. Between them, Voltaire and Rousseau largely invented the phenomenon of the intellectual celebrity, and their resulting popularity, of which even absolute monarchs took heed, gave them a form of protection that unknown figures like de La Barre tragically lacked.

Of course, both men also enjoyed a more traditional form of writers’ insurance: aristocratic patronage. Rousseau may have written the century’s most eloquent defenses of social equality, but he could never have done so without the money, shelter and protection offered him by the Duke and Duchess of Luxembourg, the Prince de Conti, the Earl Marischal, the Marquis de Girardin and many others. Had Voltaire not made close friends at school with the future Marquis d’Argenson and Duke de Richelieu, his youthful poem accusing France’s ruling regent of incest might well have been his last (as it was, the regent had him banished from Paris for it). Historians of late have given particular prominence to female patrons, who transformed intellectual conversation with Enlightenment authors into an art form. In the case of Voltaire’s liaison with the Marquise du Châtelet, and Rousseau’s with the Countess d’Houdetot, the relationship turned passionate and erotic, although in the latter case it remained unconsummated (both women, incidentally, also took as a lover the aristocratic poet Saint-Lambert; Châtelet died giving birth to his baby).

Neither Pearson nor Damrosch, both literary critics by training, engages extensively with this historical background, and as a result their books give relatively little sense of the Enlightenment as a cultural phenomenon. While both provide exhaustive accounts of the aristocratic patrons, they spend much less time on their subjects’ equally important relationships with the common reader. The books chronicle the personal dangers but largely ignore the broader intellectual reactions that the two men provoked, including a smoking lava flow of condemnation from the Christian churches. For that matter, neither Pearson nor Damrosch gives much attention to the Enlightenment as an intellectual movement, or to their subjects’ relationships with other thinkers.

Rather, the books offer conventionally framed life stories of two figures who were intellectual colleagues, rivals and ultimately bitter enemies. Pearson takes us through the familiar territory of François-Marie Arouet’s 1694 birth in the home of a Paris notary (although his true father was probably a writer of popular songs), his brilliant school career and quick success as a playwright under the name Voltaire, and the early notoriety he earned for his daring Philosophical Letters, based on his observations of England. We follow him through his prolific love affairs, his even more prolific travels, his fantastically prolific publications and his unbelievably prolific correspondence, as he transformed himself into the century’s grand, totemic intellectual. Living just across the border in Switzerland, Voltaire waged crusade after crusade for freedom of thought, finally returned to Paris for a triumphant visit at age 84 and then, with impeccably theatrical timing, died.

Damrosch, whose biography has been nominated for a National Book Award, begins in 1712 with Rousseau’s birth to a Genevan watchmaker father and a mother who died from complications resulting from childbirth. We learn of the man’s idiosyncratic early education, his running away from home before his sixteenth birthday, his subsequent picaresque wanderings, his tardy achievement of fame and his writing of some of the most influential works of the century, including The Discourse on Inequality, The Social Contract, La Nouvelle Héloïse, Emile and the Confessions. The book concludes with Rousseau’s slide into increasing eccentricity and paranoia, his break with nearly all his former friends and his death, on the gentle grounds of the château of Ermenonville, just five weeks after Voltaire’s.

In the classic Anglo-American biographical manner, both books focus heavily on personality, personal relationships and the physical settings (both therefore devote considerable space to the scenery of Switzerland, place of Rousseau’s birth and Voltaire’s exile). Pearson gives a lavish account of the landscaping and renovations carried out at Voltaire’s château in Ferney–even the heating and plumbing (“the sanitary requirements of master, mistress and guests were met by the chaise percée…water was heated in a boiler and delivered through lead piping to a tin bath lined with marble”). Damrosch includes photographs of Rousseau’s surviving abodes and describes the plaques and street signs that now adorn them. Both authors give lavish attention to clothes, notably the flowingly effeminate Armenian gown Rousseau had made for himself in the early 1760s, to the astonishment of his neighbors, and the velvet-and-ermine housecoat Voltaire wore to keep warm (it cost twice the average workman’s yearly wages). Bodily complaints naturally receive detailed treatment as well, particularly since, like nearly everyone in the eighteenth century who survived to middle age, both men suffered from painful chronic ailments. Voltaire had a notoriously weak digestive system, which he could only soothe by drinking ass’s milk. Rousseau often found it almost impossible to urinate, relying on primitive catheters that caused frequent infections and terrific discomfort.

Above all, of course, there is sex, in both cases titillatingly irregular, although for Rousseau largely in the head (“D.H. Lawrence used to preach against sex in the head, but that is where Rousseau’s sex life nearly always took place,” Damrosch comments nicely). Rousseau’s urinary affliction obviously contributed to his inactivity on this front, but as Damrosch plausibly suggests, his obsession with his penis probably had psychosomatic origins. Nor did the affliction cause complete impairment. In 1766 the young James Boswell, of all people, seduced Rousseau’s longtime companion, Thérèse Levasseur, meticulously recording that they made love thirteen times. “Don’t imagine you are a better lover than Rousseau,” she chided him, deflatingly. Thérèse, an engaging but barely literate laundress visitors routinely mistook for Rousseau’s housekeeper, may have borne as many as five of his children. But in the behavior that has caused his admirers the most pain, the philosophe deposited each newborn child at a foundling home, where the dreadful conditions almost certainly killed them.

Curiously, for both men important chapters of their sexual lives involved incest. Rousseau, after running away from Geneva, found a home with the eccentric and beautiful Madame de Warens, whom he called “mother.” Three years later the relationship, the most important of Rousseau’s life, became amorous. As for Voltaire, after the death of his great love the Marquise du Châtelet, he set up house with his own niece, Madame Denis, and in 1753 made her pregnant, although she miscarried (“would her child have called its famous progenitor ‘Great Uncle’ or just plain ‘Papa’?” Pearson asks irreverently). Despite living in this exceptionally fragile glass house, it was Voltaire who revealed the fate of Rousseau’s abandoned children in a vicious 1764 pamphlet.

Little of this material is unfamiliar to specialists. Pearson and Damrosch have both drawn principally on their subjects’ published works and correspondence, the memoirs of acquaintances and other established sources from the period. Nor does either author pretend to offer dramatically new interpretations. Damrosch, while making persuasively strong claims for Rousseau’s radical originality, relies heavily on the earlier work of Jean Starobinski and Arthur Melzer. Pearson closely follows earlier studies as well, particularly by the French Voltaire scholar René Pomeau. Both books, in other words, amount to popular distillations, more than original research.

But popularization was an honorable and important Enlightenment art. Voltaire collaborated with the Marquise du Châtelet on popular expositions of Newton’s work, and both he and Rousseau engaged brilliantly with broad general audiences throughout their careers. As popularizations both new biographies succeed marvelously. They provide full, vivid, dramatic and well-informed portraits of their subjects. And by doing so, they remind us of something professional historians of the period all too often forget: If the Enlightenment did in fact survive and flourish, it did so not only because of sweepingly impersonal changes in structures of belief, the book market and institutions of intellectual sociability but because of the actions of some remarkable individual thinkers.

Pearson and Damrosch have also managed to overcome obstacles that have defeated many previous biographers. True, neither Voltaire nor Rousseau presents the sort of problems found in a figure like Shakespeare, who died nearly eighty years before Voltaire’s birth, yet on whom we have such little information that biographers must resort again and again to little more than hopeful speculation. The works and correspondence of Voltaire fill hundreds of large, dense volumes, and while Rousseau takes up less shelf space, among his works is the astonishing autobiographical Confessions. But the sheer volume of Voltaire’s material quickly becomes overwhelming, and Rousseau’s Confessions mislead as well as inform, reflect their author’s creeping paranoia and present the constant dilemma of what to make of episodes for which there exists no independent corroboration. Pearson’s and Damrosch’s books weigh and distill this daunting source material as well as any concise biographies in English.

The books make clear that despite many surface parallels (the incest, the chronic afflictions, the persecution and exile, the aristocratic patrons), Voltaire and Rousseau were not just remarkable but remarkably different: the Janus faces of the Enlightenment. Voltaire, despite his reputation, became a radical against his own inclinations. Throughout much of his career he longed for nothing more than to ascend the established cultural heights of the Old Regime, particularly membership in the Académie Française and the post of Historiographer Royal (he succeeded in both). He ruled his Swiss domain like a lord of the manor and, in classic aristocratic fashion, treated his life as one long elegant, utterly assured theatrical display. He disdained the poor; and despite his stinging critiques of war, he made a fortune investing in military contracting. His hatred of established religion bled into a vicious anti-Semitism that went far beyond the Orientalism-tinged anticlericalism of his play Mahomet (itself mostly meant as a covert attack on the Catholic church, and understood as such at the time). He was insufferably vain. “It is not enough for him to be the hero of the century,” wrote his disciple La Harpe (in a passage quoted by Damrosch, not Pearson). “He wants to be the news of the day, for he knows that the news of the day often makes people forget the hero of the century.”

Pearson acknowledges all these points–with the exception, perplexingly, of the anti-Semitism. And he admits that Voltaire was not really an original thinker. But he compellingly argues that the man’s very lust for fame, tied to a basic sense of justice, led him into the fray, again and again, against intolerance and fanaticism, most notably in the campaign to exonerate Jean Calas. It also led him to employ his wonderfully powerful wit in an unending flow of essays, histories and philosophical tales (especially Candide) that brought the Enlightenment credo of tolerance and rationality to an entire continent of readers.

Rousseau, though no amateur when it came to vanity, could not have been more different. Neither a brilliant student nor an artistic prodigy, he struggled for decades before making his reputation. Damrosch plausibly suggests that he suffered from dyslexia, which forced him to read very slowly but also, perhaps, with unusual concentration and intensity. Although capable of great charm, he usually felt ill at ease in social situations and ostentatiously scorned the sort of glittering positions Voltaire so avidly sought, for fear of ending up in a situation of dependence. Eventually he broke with society altogether and decided to live the life of a modern hermit (in truth, a rather coddled hermit). He gave sincerity the place of honor among the virtues and longed for what Jean Starobinski famously called “transparent” relations between people, unobscured by deceitful appearances. He praised simple sentiment above complex reasoning and called religion a matter of feeling, not intellection. From his condemnations of the hypocrisy and corruption of society stem two centuries of radical critique.

Many others had made such critiques before, in the religious language Rousseau could not help echoing. What gave his version such immense power and originality was the way he tied it to the modern concepts of social and personal development. Most of the philosophes, including Voltaire, saw both sorts of development as a simple matter of overcoming ignorance and barbarity. In this view, as both individuals and societies grow older, they learn manners and refinement, adopt more rational forms of behavior and put behind them the willfulness, violence and ignorance of earlier stages of growth. These stages themselves–infancy or “primitive” society–have little inherent interest. Rousseau, by contrast, articulated the quintessentially modern idea that these stages have a formative influence on what follows. As societies develop so does mutual dependence, and with it exploitation, inequality, insincerity and unhappiness. Meanwhile, as children develop into adults, significant influences and relationships determine enduring patterns of behavior. Of Rousseau’s great works, The Discourse on Inequality and the Confessions provide classic diagnoses of social and personal development, and the origins of what we would now term collective and individual pathologies. Emile and The Social Contract, both published in 1762, offer conjectural blueprints on how to avoid these pathologies: how one might go about creating a just society, or raising an unalienated “natural” individual.

As a concrete example of the vast gulf between Voltaire and Rousseau on the subject of development, consider the way the two dealt with what modern readers would consider the most painfully formative of youthful experiences: sexual molestation. Voltaire, we learn from Pearson, once casually remarked to the mother of Alexander Pope that “if he suffered such constant ill health…it was because he had been repeatedly sodomized at school.” However, he otherwise said nothing about the experience and apparently did not consider it very significant. In the only autobiographical sketch he ever wrote, he dismissed the very idea of its significance: “Nothing is more insipid than the details of infancy and the time spent at school” (again, a statement quoted by Damrosch, not Pearson). By contrast, in a famous episode in the Confessions, Rousseau recounted how a fellow lodger at a Catholic hospice tried to seduce him, and he did not flinch from a single detail: the odor of chewing tobacco, the filthy groping hands, even the sudden spurt of something “sticky and whitish.” Precisely through the vivid analysis of such “details of infancy,” and the realization that they constitute foundation stones of the adult personality, the Confessions virtually created the genre of modern autobiography, and the narrative still stands as one of the greatest, most penetratingly introspective self-portraits ever written.

Not surprisingly, given these differences, the two men ended up loathing each other. Voltaire, characteristically, deprecated Rousseau with witty elegance: “That man is artificial from head to foot, in mind and in soul”; “He is like a child who thinks he has done something impressive when he blows soap bubbles or makes ripples by spitting into a well.” Only when Voltaire got truly annoyed did he stoop to calling Rousseau a “Jeanfoutre” (Jean-fuck) and urge that he be hanged. Rousseau, meanwhile, quite typically expressed his own dissatisfaction in more psychological language: “I do not like you, sir. You have chosen to wrong me in the ways that would cause me the most grievous hurt, and I your disciple and enthusiastic admirer…. I hate you as one who could more worthily have loved you, had you but wished it.” It’s worth noting that Voltaire loathed the novel, that most introspective of modern art forms, while Rousseau wrote the most popular novel of the century: La Nouvelle Héloïse.

Pearson and Damrosch succeed so well in conveying these points because, like their subjects, they both understand the importance of style. Damrosch, who gives an excellent précis of Rousseau’s ideas, settles mostly for a brisk, pleasantly dry tone. Still, he can turn wryly ironic on occasion–such as when contrasting Rousseau’s unceasing belief in his imminent demise (he was the century’s champion hypochondriac) with the abundant evidence of his generally robust health (penis aside). Damrosch also has a brilliant eye for quotations, and he sprinkles them through the book to great effect. For instance, Rousseau’s comment to Malesherbes on the gallows, or David Hume’s marvelous verdict on Rousseau’s increasing paranoia: “He is plainly mad, after having long been maddish.” Or Rousseau himself, creepily expressing this madness: “The ceiling above me has eyes, the walls around me have ears. [I am] surrounded by spies and by malevolent, vigilant watchers.”

Pearson, who has less to say about his subject’s less original ideas, has greater stylistic ambitions. He shifts between cinematic evocations of the setting (“the clatter of wheel on cobble, the crack of whip and driver’s curse, the hawker’s cry and the policeman’s halloo”) and passages that echo Voltaire’s style. The chapter titles come straight out of Candide, which Pearson has previously translated (e.g., “How the lawyers had themselves a lovely holocaust, and how two cuckolds turned a blind eye”). Sometimes, Pearson strives too hard for effect, as with the stale trick of calling Voltaire by different names to underline his shifting identities: “Voltaire,” “François,” the family name “Arouet,” even the toddler nickname “Zozo.” Pearson speaks twice of Frederick the Great’s “bitchiness,” when once would have been too much. But when he hits his stride he can be magnificent, as in his description of how Voltaire read Rousseau’s assertion that the splendor of dawn in the mountains demonstrates the existence of God. “Let’s see if Rousseau was right,” he announced to a sleepy houseguest in the middle of the night, and bundled the man out the door for a two-hour journey to the top of a hill:

The Jura mountains rose before them, the dark green tips of their trees etched against the pale blue light of morning, while below their feet a landscape of meadow and stream began to emerge from the crepuscular blur. And there, on the far horizon, among the pine-clad peaks, the sun rose in a vast semicircle of purple fire. Resplendent. Incontrovertible. Voltaire removed his hat and prostrated himself on the ground. Thus prone…he improvised a quasi-poetic chant of worship: “I believe. I believe in You. Almighty God, I believe.” Whereupon he got to his feet, replaced his hat, and dusted the dirt from his elegant breeches…and added: “As for Monsieur, the son, and Madame, his mother, that is quite another matter.”

The episode may never have taken place, but if it didn’t, it should have.

Both books end, not surprisingly, in the same place: the Paris Panthéon, just a few blocks from La Libre Pensée, where the French Revolutionaries transferred both Voltaire’s and Rousseau’s bodies, and where the two men now lie in an austere neoclassical crypt, just a few feet from each other. Damrosch comments on the irony of this juxtaposition (both men must be spinning madly in protest at the other’s presence), but neither biographer discusses its broader, unintended symbolism. The Revolutionaries, of course, in transferring the two bodies to this shrine of “the great men of the fatherland,” meant simply to signify the triumph of the Enlightenment, which they associated with their own political movement. And the authors undoubtedly deserved their status as the Enlightenment’s two principal French representatives: They came out of the same “enlightened” coterie of friends, contributed to the same “enlightened” projects, shared the same basic epistemological assumptions.

But in Jean-Jacques Rousseau the great critical glare of the Enlightenment, which Voltaire had fixed so scorchingly on prejudice and superstition and intolerance, was turned on itself. Critique became auto-critique, and crusade shifted into introspection. Not surprisingly, nearly all the early enemies of the Enlightenment ended up being influenced by Rousseau, often despite themselves: Romantic poets, embryonic socialists, even ultra-Catholic defenders of throne and altars (for did not the mountains at dawn proclaim the existence of God?). Even today, often without knowing it, the enemies of the Enlightenment echo him. The critique of Western civilization that Ian Buruma and Avishai Margalit call “Occidentalism,” which they find shared by everyone from the Nazis to Al Qaeda, is to a great extent a perversion of Rousseauism. The notion of the Enlightenment as cold, mechanistic, materialistic, instrumental, overly rational, devoid of sentiment or sincerity or religious feeling: All this, we owe to him. The critique, sharp and stinging, has been there from the very start. Is it any wonder, then, that the Enlightenment has been such a fragile thing?