Friedrich Nietzsche's descent into madness prevented him from achieving his dream of seeing Paris, but his philosophical alter-ego Zarathustra took the City of Light by storm, raising sharp debates among the political and cultural avant-garde about the very foundations of modern philosophy, social thought and political life. This intellectual history reveals Nietzsche's impact upon the French life of the mind and clarifies the crisis in European thought that foreshadowed and helped bring about World War I.