The influential writer known for their contributions to existential philosophy is Jean-Paul Sartre. His works, including "Being and Nothingness," explore themes of freedom, choice, and the human condition. Sartre's ideas question the nature of existence and emphasize the responsibility of individuals in creating meaning in their lives. His thoughts on existentialism profoundly impacted literature, philosophy, and psychology, encouraging people to confront their anxieties and embrace their subjectivity in a seemingly indifferent universe.