The prominent philosopher known for his theories on existentialism in the 20th century is Jean-Paul Sartre. Sartre's philosophy emphasizes individual freedom, choice, and the inherent meaninglessness of life, positing that individuals must create their own essence through actions. His seminal work, Being and Nothingness, explores these concepts, profoundly impacting literature, psychology, and philosophy. Sartre also engaged with political activism, advocating for existentialist principles in the context of social and political movements. His influence continues to shape contemporary thought on existence and morality.