Beauvoir placed second in the 1929 agrégation in philosophy, at twenty-one, the youngest person ever to pass it. Sartre placed first, on his second attempt. The jury discussed which of them was the real philosopher and several members said it was her.

Then she spent fifty years being filed as his follower, including by people who had read both.

Her actual position

The difference from Sartre is not a matter of emphasis, and it is the reason to read her.

Sartre's freedom is absolute: the situation constrains what you can achieve and never touches your freedom to interpret it. Beauvoir's freedom is situated — a real capacity that a situation can genuinely diminish, and that other people can be prevented from having.

The Ethics of Ambiguity (1947) works this out. Freedom is not a fact about consciousness to be acknowledged; it is something to be won, and it depends on conditions. A person kept in ignorance, or in dependence, or in a role with no exits, has less of it — not less power to act on it, less of it. Which makes my freedom depend on yours, because a world organized to deny freedom to some people is one in which nobody's is secure.

That is an ethics, which Sartre promised at the end of Being and Nothingness and never delivered. She wrote it in two years.

The lived body

Her other departure, taken from Merleau-Ponty and used better.

The body is not an object you own and not a constraint on a free consciousness. It is the situation through which everything is experienced — and it is sexed, aged, healthy or ill, and read by other people before you speak. A philosophy that treats consciousness as bodiless has quietly assumed a body that nobody remarks on, which in 1949 France meant a man's.

This is the move that makes The Second Sex philosophy rather than sociology, and it is why the book spends hundreds of pages on adolescence, marriage and aging. Those are not illustrations. On her account they are where the situation is made.

Ambiguity

The word in her title is the position. A human being is both a free consciousness and a thing among things — subject and object, at once, with no resolution. Most ethical systems pick one and build from it, and she thinks both moves are evasions.

What follows is an ethics without guarantees. You cannot derive what to do from what you are, because what you are is contradictory. You act anyway, the outcome is not underwritten, and the willingness to proceed on those terms is what she means by adulthood.

The objection

She and Sartre had sexual relationships with her students, some of them teenagers, and the pattern was sustained.

Beauvoir was suspended from teaching in 1943 after a complaint from the mother of Nathalie Sorokine, and lost her license. Bianca Lamblin, who had been her pupil at seventeen, published an account in 1993 describing being drawn in by Beauvoir and passed to Sartre, and the correspondence published after Beauvoir's death corroborates the outline and the tone.

There is no reading of this on which a philosopher who argued that freedom depends on the conditions other people are placed in was not, personally, arranging bad conditions for people young enough to be taught by her. The argument in the books is not damaged by it — arguments are not — and the biography does not become a footnote either.

Why the misfiling lasted

Partly her own doing: she described herself as not a philosopher and as working within Sartre's system, and interviewers took it at face value for decades.

Mostly not. The Second Sex was read as feminism and therefore as not philosophy, a category judgment that did the work of an argument. The recovery began in the 1980s and is now settled — The Ethics of Ambiguity predates Sartre's turn by more than a decade, and the situated account of freedom is in her work before it is in his. See existentialism for where both of them sit.