The legend of Simone de Beauvoir—of how an obedient Catholic schoolgirl cast off her rigid, patriarchal upbringing to become the high priestess of existential feminism—is often narrated as a love story. Her biographers trace her escape from the bourgeois Parisian milieu into which she was born, in 1908, first to the Sorbonne and then to the École Normale Supérieure. There, among the “graceless faces” of the agrégation candidates of 1929, she spied Jean-Paul Sartre, twenty-four years old and—as she rhapsodized in “Memoirs of a Dutiful Daughter” (1958), the first of four autobiographical volumes—“still young enough to feel emotional about his future whenever he heard a saxophone playing after his third martini.” Together, she and her “Playboy,” her “Leprechaun,” as she called him, chased life’s pleasures up the steps of Boulevard du Montparnasse, down the Avenue d’Orléans, and all around the woodland parks of Paris, where her parents had forbidden her and her sister to speak with children outside their social class. Beauvoir’s mother was devoted to the Church and its rigid moralism; her father detested intellectuals and wanted his oldest daughter to “marry a country cousin.” By the time she met Sartre, Beauvoir had different aspirations. “Never have I so loved to read and think, never have I been so alive and happy or envisioned a future so rich. Oh! Jean-Paul, dear Jean-Paul, thank you,” she wrote in her diary.

For all the romance of the city blooming before her eyes, Beauvoir always played the love story itself—her dawning attraction to this garrulous, cross-eyed, funny little man—remarkably cool. She liked Sartre’s face and company but disliked his “false eye.” She was more painfully and earnestly attracted to her friend the philosopher René Maheu, and even to her rakish cousin Jacques. But she exulted in Sartre’s attention to her; it was an opportunity to define herself and the force she longed to be in the world. “We used to talk about all kinds of things, but especially about a subject which interested me above all others: myself,” she wrote. “Whenever other people made attempts to analyse me, they did so from the standpoint of their own little worlds, and this used to exasperate me. But Sartre always tried to see me as part of my own scheme of things, to understand me in light of my own set of values and attitudes.” With him, she was not “the Other,” the subordinate female position that she described in her 1949 feminist classic, “The Second Sex”: a timid and imaginatively impoverished creature for whom love was simply something provided to a husband as a matter of course. Beauvoir’s relationship with Sartre, as she recounts it in her autobiographical writings, led her from a state of alienation, in which she was only a bit player in other people’s “little worlds,” to the assurance that she was a singular and irreducible “Self.” She could define and interpret the meaning of her life through the energetic exercise of her “sovereign consciousness.”

Beauvoir’s autobiography, in a sense, accorded perfectly with the principles of French existentialism—its insistence on the freedom of every person’s consciousness and its Sartrean slogan that “existence precedes essence.” Perhaps the correspondence is a little too neat. True, intellectuals often pride themselves on living and loving by their theories—“There is no divorce between philosophy and life,” Beauvoir famously avowed—but it is hard to believe the story of their early days quite as she tells it. Not a hair is out of place, not a moment of shock or doubt ripples the surface of her triumphant self-determination. “Every woman in love recognizes herself in Hans Andersen’s little mermaid who exchanged her fishtail for a woman’s legs for love, and then found herself walking on needles and burning coals,” she claims in “The Second Sex.” How to reconcile such self-abnegating masochism with her joyous recollection of discovering herself with Sartre?

As careful readers of Beauvoir’s memoirs and diaries have noted, the strategically plotted romance between postwar France’s greatest male and female philosophers is haunted by the presence of a shadowy third—a friend. The most extreme feelings of agitation and rapture are reserved not for Sartre but for Elisabeth Lacoin, a brilliant and mercurial classmate whom Beauvoir called Zaza, and whose name Beauvoir underlined in black or brown ink throughout her diaries, casting a pall over all that surrounded it.