 Four existentialist thinkers - Kierkegaard, Nietzsche, Heidegger, and Sartre - receive extended discussion from Barrett, who explains their main ideas and philosophical terminology.Barrett more briefly discusses other existentialist thinkers such as Karl Jaspers, Nikolai Berdyaev, Martin Buber, Miguel de Unamuno, Gabriel Marcel, Simone de Beauvoir, and Albert Camus, as well as some artists and writers he considers existentialist, such as the Russian novelist Fyodor Dostoyevsky, whom he compares to Nietzsche, crediting him with anticipating Nietzsche's insights into the will to power in "Crime and Punishment" (1866).Discussing Kierkegaard, Barrett maintains that the Danish philosopher is relevant even to non-Christians because of his "appeal to their own existence."