It’s probably the most quoted slogan in feminism and certainly among the best-known theses in all philosophy: ‘On ne naît pas femme: on le devient.’ On the dust jacket of Kate Kirkpatrick’s incisive and compelling biography of Simone de Beauvoir, this appears as ‘One is not born a woman, but becomes one’. Most English readers will recognise the 1953 translation by H M Parshley better: ‘One is not born, but rather becomes, a woman.’ Either way, it’s become a talisman in the nature–nurture debate, a badge of blank slate-ism, the only line of Beauvoir’s work most people ever refer to and one of the most woefully abused sentences ever to have been written.

Nowadays, ‘One is not born…’ is regularly recruited to support the extraordinary (but ascendant) position that whether one is considered a man or a woman should be determined by personal fiat, regardless of one’s sex. This misapplication goes back to Judith Butler’s 1990 Gender Trouble, in which the author at least had the decency to acknowledge that in using Beauvoir to argue that ‘gender is not tied to sex’, she was arriving at a conclusion that Beauvoir ‘herself did not entertain’. Subsequent appropriators have been even less careful, their ignorance of the rest of Beauvoir’s output reassuring them that she meant what they hope she meant.

Where Beauvoir would fall in the vexed contemporary debate among feminists over gender identity is a hypothetical question, and although Kirkpatrick briefly addresses the afterlife of Beauvoir’s ideas, she sensibly leaves that particular wasps’ nest alone. There is, however, a strong suggestion that Beauvoir would not have been on board with the current vogue for deleting the word ‘woman’ in favour of desexed formulations such as ‘menstruator’ or ‘pregnant person’. The suggestion comes from the fact that when 20th-century American feminists tried this, Beauvoir declared them to be full of crap (or, as she actually put it, ‘bad faith’):

Women like Dorothy Parker thought the inequality between the sexes could be resolved by defining women as ‘human’

instead of ‘woman’. But the problem with the ‘we’re all human’ point of view, Beauvoir said, is that women are not men. The equality they share at this level is abstract – and the possibilities open to men and women are different.

Formal equality means very little in practice when the world has been produced entirely on men’s terms. So long as men are considered the ‘default category’ (and, as Caroline Criado Perez forensically demonstrated in Invisible Women earlier this year, they still are, seventy years after Beauvoir pointed out in The Second Sex that woman is the ‘other’), the word ‘person’ is implicitly understood as indicating maleness, and women are excluded from it unless specifically named. It’s a condition that Beauvoir knew intimately in her own life. She lived with resolute self-determination, in defiance of the limits placed on her sex, but was nonetheless ‘treated as a woman’ – inferior, secondary and derivative.

The most obvious way in which Beauvoir has been ‘treated as a woman’ is in the handling of her relationship with Sartre. Their longstanding alliance of affection and intellect led to her being seen as his disciple. Her fictional works (Beauvoir achieved fame first of all as a novelist with the autobiographical She Came to Stay, originally published in 1943) were judged as ‘thesis novels’ whose ‘theses could be found in Sartre’s philosophy’. When he died, she was barely mentioned; when she died six years later, Sartre dominated obituaries of her. And he has been presented as the architect of their non-monogamous ‘pact’, with Beauvoir cast as a beleaguered ‘wife’, tolerating his infidelities with the surrounding ‘family’ of ‘contingent’ lovers.

In Kirkpatrick’s work, Sartre appears far more in need of Beauvoir than Beauvoir is of him. In his later life in particular, it is hard to see him as anything other than pathetic: ruined by alcohol and amphetamines, throwing himself down the intellectual dead-end of Maoism, still skirt-chasing even after several strokes. Much of what is considered to be Sartre’s unique contribution to philosophy and ethics turns out – on consultation of Beauvoir’s letters, diaries and published work – to have started with her.

That goes for the ‘pact’ too. Before the two had discussed such a thing, Beauvoir ‘came to the conclusion that she would love multiple men in the ways she thought them loveable’, writes Kirkpatrick. When the two first met, as students in the late 1920s, Beauvoir may already have had a lover. The record is hazy, understandably so given the censoriousness shown towards unmarried, sexually active women in the early 20th century. Public attitudes also explain why in her memoirs she encouraged the perception that Sartre had led and she had followed, at least in their sex lives. Even writing about female sexuality in impersonal terms in The Second Sex got her slammed in the press as a ‘female follower of Bacchus’ and ‘nauseating’.

She loved other men, but she was loyal to Sartre, whom she called ‘the incomparable friend of my thought’, even at the end trying to wrest him from entanglements with manipulative young lovers and friends who wanted to hitch themselves to his reputation. Recently published letters reveal, however, that she wrote sharply about the shortcomings of their union, both ethical and sexual (he was apparently not much in the sack). She was unsparing of herself when her and Sartre’s treatment of contingents fell short, and, in relation to one young woman in particular, ‘Beauvoir judged her own behaviour “disgraceful”.’

Kirkpatrick does Beauvoir a great service in revealing these self-criticisms, because Beauvoir did not address the episodes that led to them directly in her own writing (either to protect her lovers or to protect herself). Her and Sartre’s sexual relationships with young women in the 1930s (some of them Beauvoir’s former students) were only revealed in detail after her death and were at first reported as though she felt no repentance for the harm they did. Beauvoir’s private reckoning with her own behaviour was integral to the development of her ethics, argues Kirkpatrick.

In some ways, then, Beauvoir helped to distort her own reputation through omission. In others, it was distorted for her – most shockingly in Parshley’s translation of The Second Sex, which cut nearly 15 per cent of the original, systematically removing the feminism from her argument. That disfigured document is still all that most people know of her writings, and yet she considered her fiction perhaps the most important representation of her philosophy (the ambiguity of story, she argued in her 1946 essay ‘Literature and Metaphysics’, allowed readers freedom, which she valued highly), while her memoirs provoked the most passionate responses among her fellow Frenchwomen. In Kirkpatrick’s biography, Beauvoir is restored to her full body of work, her full complexity, her full bravery – so much more than one misquoted line.