Every subject defines itself against an Other — a relation in which the subject is the positive, essential term and the other is derivative, secondary, inessential. For centuries, de Beauvoir argues, this relation has structured the relation between men and women. Man is the human; woman is the deviation. Man is the norm; woman is the exception.
This otherness is not natural but constructed. Through education, socialization, and culture, girls are taught to be passive, accommodating, and dependent — to be the Other. De Beauvoir's famous formulation — "one is not born, but rather becomes, a woman" — means that the feminine is a social achievement, not a biological given.
This insight anticipates by decades the distinction between sex and gender that would become central to feminist theory. Biology provides the raw material; society shapes it into femininity.
De Beauvoir's analysis of women as Other draws on Hegelian dialectics and Sartrean existentialism. The relation she describes — Subject and Other — mirrors Hegel's master-slave dialectic, which itself influenced Nietzsche's analysis of ressentiment.