The influential woman who wrote "The Second Sex", a foundational text of modern feminism, is Simone de Beauvoir. Published in 1949, this work critically examines the oppression of women and explores themes of existentialism and identity. De Beauvoir's thesis posits that one is not born a woman but rather becomes one, highlighting the role of society and culture in shaping gender roles. This groundbreaking analysis helped ignite the feminist movement and remains relevant in discussions about gender equality.