'The Second Sex' is a groundbreaking feminist work published by Simone de Beauvoir in 1949 that explores the construction of women's identities and their oppression in a patriarchal society. This text is essential for understanding the roots of feminist theory, as it critiques the historical treatment of women and calls for their liberation through self-definition and autonomy. Beauvoir's famous assertion that 'one is not born, but rather becomes a woman' emphasizes the social constructs surrounding gender roles and has influenced movements advocating for gender equality.