The influential thinker known for her work in existential philosophy is Simone de Beauvoir. She is renowned for her contributions to feminist theory and existentialism, particularly through her seminal work, "The Second Sex." De Beauvoir's writings explore the nature of existence, freedom, and the construction of gender identities. She challenged the traditional views of women's roles in society and argued for their liberation. Her philosophical ideas advocate for individual autonomy and responsibility, making significant strides in both philosophy and feminism, leaving a lasting impact on contemporary thought.