Beauvoir and Sartre met in the summer of 1929, when they were in Paris studying philosophy for the highly competitive aggrégation exams. Beauvoir was in the middle of an unconsummated romantic affair with a married man, René Maheu, a prettily handsome boy with a soft, feminine mouth and dreamy eyes. Maheu had a study group with his friends Jean-Paul Sartre and Paul Nizan, and he brought the clever Simone along to it. He did this reluctantly, for Sartre, despite his diminutive stature and his bad squint was known for his ability to seduce women through words and ideas. By the time the exams were over, Beauvoir was hypnotised by Sartre’s vivacity, his determination, his plans and projects, and whilst Sartre frankly admired her handsome good looks, he recognised in her singular mental agility an intellect that was the match of his own.

Sartre was not offering marriage…. But he was offering her the gift of a companionship she had never imagined to receive: intellectual adventure alongside a man who would be mentor, lover and friend.

Beauvoir’s mind was outstanding. When she passed the aggrégation exams in philosophy she was, at twenty-one, the youngest ever to do so. She came second to Sartre, a mere two points behind him in marks, yet she had only been studying philosophy for three years, compared to his seven. Much later, it emerged that the examiners had debated long and hard over whom should be awarded first place in the competition, as Beauvoir had impressed them so. In the end, lame excuses gave Sartre the prize; he was the boy from the prestigious École normale, and this was his second attempt at the exams.

For Beauvoir, it scarcely mattered; association with Sartre and his friends had opened up her quiet and studious life to more fun than she had ever known before. Once their working day was done, the men would sing and joke and chat, play jazz records and visit the nearby funfair. “How cramped my little world seemed beside this exuberantly abundant universe!” she wrote. Her upbringing had been sheltered and constrained in a way that seems impossible to twenty-first century eyes. When she entered a café for the first time, aged twenty, she felt it was an act of thrilling rebellion. Both her serious study and her tendency to atheism were considered scandalous. In the aftermath of the First World War, there was considerable pressure on young women to marry and have children; contraception was illegal and refusing motherhood was an unpatriotic act.

The collapse of her family’s finances provided her with what she would consider a stroke of magnificent good luck. “You girls will never marry,” her father told Simone and her sister, aware that he would not be able to provide the necessary dowry. “You’ll have to work for a living.” Simone decided to become a teacher, and even that decision was delayed by a year after the nuns at her convent school warned her mother that “the study of philosophy mortally corrupts the soul.” Once she had finally been sprung from her gilded bourgeois cage, Beauvoir embraced the opportunity for serious study with joy. Her only regret lay in her clear-sighted assessment of her chances for marriage. “I am too intelligent, too demanding, and too resourceful for anyone to be able to take charge of me entirely,” she wrote rather sadly in her youthful journal. “No one knows me or loves me completely. I have only myself.”

Sartre was not offering marriage. He believed he was destined to be a great writer, which he conflated with the image of the lone male hero; his freedom was essential to him. But he was offering her the gift of a companionship she had never imagined to receive: intellectual adventure alongside a man who would be mentor, lover and friend. The shameful nature of their liaison would have made most women baulk, in an era when virtue was the condition for respect. But Beauvoir swiftly agreed, surprising Sartre. To find a man as ambitious for her as she was for herself must have felt miraculous. She wrote in her diary, “Never have I been so alive and happy, or envisaged such a rich future. Oh Jean-Paul, dear Jean-Paul, thank you!”

Never could she have envisaged how much determination and stoicism her future would require from her, either. Sartre dictated the terms of their relationship according to his desire to have his cake and eat it. He felt strongly that love was not about possession, that it should be generous and benevolent, not restricting in any way. So whilst theirs would be the “essential” love, they should not deny themselves the pleasure of “contingent” affairs with other people. Sartre had the brains to express this as a figment of personal philosophy, but it was inevitably self-serving. He wrote in his diaries that he longed to be “a scholarly Don Juan, slaying women through the power of his golden tongue” and throughout his life he displayed a mania for chasing women single-mindedly up to the point when they gave in to him. He confessed that “nothing struck me as more moving than the moment at which the avowal of love is finally wrenched forth.” After that, he rarely knew what to do with his conquests. But both he and Beauvoir had a singular inability to move on from lovers, insisting instead on binding them as permanently as possible into their lives.