Only those who lived before the French Revolution, said Talleyrand, could know the true sweetness of life. Reading “Germaine de Staël and Benjamin Constant” (Yale University Press, 343 pages, $35), the new “dual biography” by Renee Winegarten, suggests that the notoriously wily and corrupt diplomat was at best half-right. Constant, the liberal philosopher and politician, and de Staël, whose fame as a writer, wit, and opponent of Napoleon extended from London to Petersburg, were old enough to remember the days before the Bastille fell. When the couple met, in 1794, she was 28 and he was 26; they would remain the center of each other’s lives, despite numerous betrayals and breakups, until de Staël’s death in 1817.

Yet while they lived in a post-revolutionary world, their love affair, as evoked by Ms. Winegarten in this fast-paced and entertaining book, had something of the grandeur and douceur of the ancien régime. It was a courtship carried out in country estates and Paris drawing rooms, at roulette tables (Constant was an incorrigible and usually unsuccessful gambler) and sparkling salons (de Staël’s admirers included just about every man of genius of the age, from Goethe to Talleyrand himself). No Versailles dinner table could have offered more spectacular conversational duels than the ones de Staël laid on at Coppet, her estate outside of Geneva.

And the star combatants were always Germaine and Benjamin, a Beatrice and Benedick for the age of Romanticism. “By means of a mind equal to hers,” remembered one of their friends, “he alone had the power to put all her intelligence into play, to make her grow more great through their combat, to awaken an eloquence, a depth of spirit and thought that were never manifest in all their brilliance except opposite him, just as he was never truly himself except at Coppet.”

Their attitudes toward fidelity, too, were old-school. Though neither was a true aristocrat — both were Swiss Protestants, descended from Huguenot exiles — they were as addicted to seduction and betrayal as any grandee in “Les Liaisons Dangereuses.” When they first met, at a late-summer party at the home of Constant’s cousin, it was Benjamin who took the initiative, Germaine who played defense. (Appropriately, since she is writing history as a form of higher gossip, Ms. Winegarten uses their first names throughout.)

It was not that he found her extraordinarily beautiful. “She was dark where the beauties of the day were fair,” as Ms. Winegarten puts it, “her build was stocky rather than svelte, her movements forceful rather than elegant.” But Benjamin, like her many other conquests, found that Germaine’s passion and intelligence more than made up for her unconventional looks. Nor did it hurt that she was very rich, the only child of the legendary financier Jacques Necker, who had served as chief minister to Louis XVI before the revolution. Within a month of their first meeting, Benjamin was writing to friends that Germaine “is a Being apart, a superior Being, one of a kind perhaps to be found in a hundred years, so that those who associate with her, know her, and count themselves her friends, should not ask for any other form of happiness.”

He might count himself her friend, but it was a year and a half before he became her lover. For while de Staël immediately appreciated Constant’s genius, she made it plain that she was not at all attracted to him. She described him in letters to her then-lover, a Swedish nobleman named Ribbing, as “outstandingly ugly,” though “exceptionally intelligent.” Soon she was writing, with a mixture of gratification and disgust, “M. Constant … has conceived a passion for me that is beyond description, he is at death’s door and overwhelms me with so much misery that it deprives him of his one charm — a very superior mind.”

Indeed, Constant’s idea of wooing involved smashing his head against the mantelpiece and attempting suicide with opium, all in an attempt to prove to de Staël that he could not live without her. (Naturally, he did not take enough opium to actually kill himself, leading an acquaintance to remark, in acerbically French fashion, “When you play jokes of that sort, you should die of it, for the honor of the lady.”) This did not discourage her from taking Constant to Paris as part of her entourage, setting him up with money and property, and launching his career as a political journalist. De Staël knew talent when she saw it, and, as Ms. Winegarten points out, sponsoring a great man was one of the only ways for a woman to gain political influence. Finally, after de Staël’s other love affairs sputtered out, she succumbed to Constant’s long siege. In early 1796 they became lovers, and remained a very public couple for the next 12 years.

Not that they were always together, or ever exclusive. Indeed, both of them were married when they first met. Benjamin was in the process of divorcing his first wife, an undistinguished older woman whom he had married while serving at a petty court in Germany. Germaine, on the other hand, had no intention of leaving her husband, Eric de Staël, whose position as Swedish ambassador to France gave her a lifesaving diplomatic immunity during the turmoil of the revolution. This advantage more than made up for his stupidity — one acquaintance remarked that Eric “would not be capable of inventing a dish of potatoes.”

But neither Constant nor de Staël felt bound to their spouses: marriage, for people of their time and class, was a business arrangement, not an intimate tie. Then again, each of them also had a liaison to bind them. Constant was involved with Charlotte von Hardenberg, who would later become his second wife, and de Staël with Ribbing, the latest in a series of handsome aristocrats. Even after they became lovers, Benjamin and Germaine kept up a busy schedule of secondary and tertiary affairs.

As Ms. Winegarten shows, Constant had no compunction about taking de Staël’s money and using her connections while pursuing a series of other women, including her best friend, the famous beauty Madame Recamier (whose portrait was painted by David). Then again, if Constant had a weakness for actresses, de Staël was equally fond of young soldiers. As Ms. Winegarten notes, Germaine could write to one of them, Maurice O’Donnell, “You who are my life … I can no longer live without you,” just a week after telling Constant “my heart, my life, everything is yours if you want it.”

“What to believe?” Ms. Winegarten asks. But while she pursues each twist and turn of the lovers’ relationship with a curiosity bordering on concern, it is not actually too hard to understand what drove Constant and de Staël. Their fondness for grand romantic gestures — the weeping fits, jealous scenes, eternal vows (sometimes committed to writing, in the form of a contract), suicide attempts — was entirely in keeping with the Rousseauian cult of sentiment. Even the way we make love, Ms. Winegarten shows, is culturally determined. It makes sense that each of them wrote a novel about their relationship — hers was titled “Corinne,” his “Adolphe” — since the whole thing had always been a kind of performance, staged for the enjoyment of the lovers themselves.

And like Rousseau, who doted on his vices as lovingly as if they had been virtues, Constant and de Staël allowed their intense focus on their own emotions to block out any consideration for the feelings of others. The heartlessness of Constant’s treatment of de Staël in money matters, for instance, or the way he disclaimed all responsibility for their daughter Albertine (he believed she was his child, though legally she was Eric de Staël’s), sits oddly with his reputation as a humanitarian crusader.

Yet it remains true that, in politics, Germaine and Benjamin were almost always on the right side — defending freedom of speech and the press, opposing the terror of the Jacobins as well as the despotism of Napoleon. If Ms. Winegarten had dwelled more on their public careers and their writings, rather than their private lives, Constant and de Staël might have emerged as more admirable figures. But then, her book wouldn’t have been so entertainingly scandalous.

akirsch@nysun.com