Jean-Paul Sartre preferred the company of women. As one would expect of the great advocate of transparency, he discussed his reasons frankly. “First of all, there is the physical element. There are of course ugly women, but I prefer those who are pretty,” he explained in an interview for the documentary “Sartre by Himself.” “Then, there is the fact that they’re oppressed, so they seldom bore you with shop talk. . . . I enjoy being with a woman because I’m bored out of my mind when I have to converse in the realm of ideas.” “Sartre by Himself” was filmed in 1972, when Sartre was sixty-six; his interviewers were loyal associates from the journal he founded after the war, Les Temps Modernes. None of them encouraged him to expand on the topic, since Simone de Beauvoir was present, and everyone in the room understood that the legend of their relationship was in her keeping. But everyone in the room also knew that Sartre liked the company of women because he devoted much of his time to the business of seducing them.

The nature of Sartre and Beauvoir’s partnership was never a secret to their friends, and it was not a secret to the public, either, after they were abruptly launched into celebrity, in 1945. They were famous as a couple with independent lives, who met in cafés, where they wrote their books and saw their friends at separate tables, and were free to enjoy other relationships, but who maintained a kind of soul marriage. Their liaison was part of the mystique of existentialism, and it was extensively documented and coolly defended in Beauvoir’s four volumes of memoirs, all of them extremely popular in France: “Memoirs of a Dutiful Daughter” (1958), “The Prime of Life” (1960), “Force of Circumstance” (1963), and “All Said and Done” (1972). Beauvoir and Sartre had no interest in varnishing the facts out of respect for bourgeois notions of decency. Disrespect for bourgeois notions of decency was precisely the point.

Sartre and Beauvoir had met in Paris in 1929, when he was twenty-four, she was twenty-one, and both were studying for the agrégation, the competitive examination for a career in the French school system. Beauvoir was a handsome and stylish woman, and she had a boyfriend, René Maheu. (It was Maheu who gave her her permanent nickname, le Castor—the Beaver.) But she fell in love with Sartre, once she got over the physical impression he made. Sartre was about five feet tall, and he had lost almost all the sight in his right eye when he was three; he dressed in oversized clothes, with no sense of fashion; his skin and teeth suggested an indifference to hygiene. He had the kind of aggressive male ugliness that can be charismatic, and he wisely refrained from disguising it. He simply ignored his body. He was also smart, generous, agreeable, ambitious, ardent, and very funny. He liked to drink and talk all night, and so did she.

Sartre had been engaged, though the engagement was broken off after he failed his first attempt at the agrégation; but he and Beauvoir decided that their love did not require marriage for its consummation. “The comradeship that welded our lives together made a superfluous mockery of any other bond we might have forged for ourselves,” Beauvoir explained in “The Prime of Life”:

One single aim fired us, the urge to embrace all experience, and to bear witness concerning it. At times this meant that we had to follow diverse paths—though without concealing even the least of our discoveries from one another. When we were together we bent our wills so firmly to the requirements of this common task that even at the moment of parting we still thought as one. That which bound us freed us; and in this freedom we found ourselves bound as closely as possible.

Sartre proposed a “pact”: they could have affairs, but they were required to tell each other everything. As he put it to Beauvoir: “What we have is an essential love; but it is a good idea for us also to experience contingent love affairs.” Beauvoir’s whole life to that point had been an effort to escape from the culture of her family. Her mother had been educated in a convent; her father was a conservative Paris lawyer of diminished means who, though he was proud of his daughter’s mind, discouraged her interest in philosophy, and would probably have discouraged her pursuit of any career if he had been able to provide her with a dowry. So she was excited by the affront to conventional standards of domesticity that Sartre’s arrangement posed. She also had a high opinion of Sartre’s brilliance as a philosopher. An argument based on terms like “essence” and “contingency” worked as well on her as a diamond ring. She saw (before he did, but in some ways she was cannier than he was) that the pact bound to her for life a man whom she knew would never be faithful. It closed the normal exit.

As matters worked out, the pact meant that Beauvoir not only discussed with Sartre his interest in other women; she often formed intimate friendships with the women herself. At first, she was distressed to discover that she sometimes felt jealous. Sartre advised her that jealousy, like all passions, is an enemy of freedom: it controls you, and you should be controlling it. Sartre soon stopped sleeping with her, and she had her own serious affairs, notably with Nelson Algren, a transatlantic relationship that lasted from 1947 to 1951, and Claude Lanzmann, with whom she lived from 1952 to 1959; she wrote openly about her relations with both men in “Force of Circumstance.” But she remained committed to Sartre and to the pact; and the relationship, with its carrousel of changing partners and café tables, lasted fifty-one years.

Beauvoir never pretended that her memoirs told the whole story. “There are many things which I firmly intend to leave in obscurity,” she warned in “The Prime of Life.” Though she strategically employed pseudonyms in the memoirs, enough was revealed, and enough suggested in her romans à clef “She Came to Stay” (1943) and “The Mandarins” (1954), to satisfy most curiosities. Sartre died, after a prolonged debilitation, in 1980. A year later, in a book called “Adieux: A Farewell to Sartre,” Beauvoir published a series of “conversations” with Sartre that she had conducted in 1974, in which she guided him through philosophically tinged musings on his affairs. Even for existentialists, it was painful reading: