Simone de Beauvoir’s feminist philosophy primarily focuses on the concept of existentialism and the construction of womanhood. In her landmark work "The Second Sex," she explores how women have been historically defined in relation to men, advocating for the idea that "one is not born a woman, but rather becomes one." Beauvoir emphasizes the importance of freedom and choice, arguing that women should define their own identities beyond societal expectations. Her philosophy challenges traditional gender roles and calls for women's liberation, significantly influencing feminist thought and activism in the 20th century.