How did Being and Nothingness establish Jean-Paul Sartre's reputation?

Being and Nothingness (L’être et le néant, 1943) established Jean-Paul Sartre’s reputation as a philosopher of remarkable originality and depth by providing a rigorous foundation for his existentialist thought.1

In this treatise, Sartre utilized the phenomenological method to distinguish between two modes of being: the "for-itself" (human consciousness) and the "in-itself" (the reality of things). By defining human consciousness as "no-thingness" (néant)—which is not matter and therefore escapes determinism—he articulated a philosophy centered on human freedom and the necessity of individual responsibility.123

Following the Liberation of France, these ideas resonated deeply with a postwar generation, helping to make Sartre a central figure in the intellectual life of Saint-Germain-des-Prés. His work, alongside his plays, set the tone for the era, cementing his status as a leading voice in 20th-century philosophy and literature. While the work offered a hopeful message regarding human freedom, its insistence that human endeavor is ultimately useless also imbued it with a tragic quality that defined his existentialist outlook.312