In 2008 we celebrate the centenary of the world’s most profoundly influential feminist philosopher. Humanists have better reasons than most to remember her, for few women (and men) have better incarnated the radical challenge of free thought than Simone de Beauvoir.

Two of Beauvoir’s books should be on the reading list of every humanist. The first volume of her autobiography, Memoirs of a Dutiful Daughter (1958), tells the compelling story of her efforts to break free from her stiflingly conventional childhood and youth. Brought up to follow the usual path for a French Catholic girl of good family – religious devotion, marriage, and children – she insisted instead on studying that most secular of subjects, philosophy, and finally escaped. By contrast, Zaza, her best friend and the subject of some of the most moving pages in the book, was not so lucky. Zaza died at 22, broken down by the conflict between her mother’s ferocious Catholic values and her own wish for a freer life.

“Some very old bonds in my life have never been broken,” Beauvoir wrote, looking back at her life: “the place that Sartre always had in it, and my fidelity to my original project: to know and to write.” Beauvoir and Jean-Paul Sartre met while they were both still students and quickly became a highly unconventional couple, refusing to marry or to have children, yet always remaining deeply committed to a joint life of work and thought.

Together, Beauvoir and Sartre developed existentialism, a philosophy that takes freedom to be the highest value in a universe where God is dead and where human beings create their own values through their choices and actions. In the postwar period, existentialism was the most challenging expression of radical, secular, philosophical humanism.

The other book by Beauvoir that all humanists should read is The Second Sex, her magnificent essay on the oppression of women from 1949, when existentialism was at its peak. Taking as its starting point the existentialist commitment to freedom, Beauvoir’s epochal book was written as much for men as for women. Beauvoir (always an optimist) thought that every open-minded human being would be shocked to learn that one half of humanity was systematically deprived of fundamental forms of freedom, and wish for change.

How wrong she was! The Second Sex was received as a sexual, political and religious scandal. One of her best friends, Albert Camus, thought she had made the French male look ridiculous. The Communists polemicised against her. The Vatican immediately placed it on the Index of books good Catholics were forbidden to read.

To read The Second Sex today is to discover precisely the kind of feminism the world needs now. Beauvoir is not interested in identity politics. She doesn’t believe in women’s ineffable “difference”. Nor does she think that women are the blameless victims of men. (The epigraph of the second part, which deals with women’s lived experience, is “Half victims, half accomplices, like everyone else.”) She does think, however, that women are systematically oppressed by a worldview which takes man to be the One, the Subject, and turns women into the Other, the Object, a relative being.

The Second Sex is an unbelievably passionate and energetic book. For Beauvoir, the future is wide open, and freedom within reach: “The free woman is just being born,” she optimistically concludes. The Second Sex urges us to have faith in our power to transform the future for the better. It should be required reading not just for humanists but for everyone who thinks that this world should be as free and welcoming to women as to men.