Philosophers with existentialist sensibilities can be found as far back as ancient times. Existentialist way of thinking has been identified in Heraclitus, Socrates, and even Augustine. Modern existentialism is generally identified with the thought of five thinkers: Søren Kierkegaard, Friedrich Nietzsche, Martin Heidegger, Albert Camus, and Jean-Paul Sartre. The wide difference in the religious and political thought of these five philosophers is something that makes it impossible to see existentialism as a school of philosophy. Modern existentialism is not a set of doctrines; it is a movement based on certain sensibilities regarding individualism and human freedom. Kierkegaard was profoundly religious, whereas Nietzsche and Sartre were atheists. Kierkegaard would have nothing to do with politics, he was disgusted by it, but Sartre, Camus, and Heidegger were not only political theorists but also political activists. Sartre was a Marxist; Camus was a staunch anti-Marxist and identified as a humanitarian; Heidegger, it is alleged, was close to Nazism. The philosophy also includes popular authors like Fyodor Dostoevsky and Franz Kafka.