The idea of the absurd is a common theme in many existentialist works, particularly in Camus. Absurdity is the notion of contrast between two things. As Camus explains it in The Myth of Sisyphus:

The absurd is born out of this confrontation between the human need and the unreasonable silence of the world.

This view, which is shared by Sartre, is that humanity must live in a world that is and will forever be hostile or indifferent towards them. The universe will never truly care for humanity the way we seem to want it to. The aethist view of this statement is that people create stories, or gods, which in their minds transcend reality to fill this void and attempt to satisfy their need.

The philosophy that encompasses the absurd is referred to as absurdism. While absurdism may be considered a branch of existentialism, it is a specific idea that is not necessary to an existentialist view.

It's easy to highlight the absurdity of the human quest for purpose. It's common to assume that everything must have a purpose, a higher reason for existence. However, if one thing has a higher purpose, what is the reason for that purpose? Each new height must then be validated by a higher one. This evokes the common theological question: if humankind was created by God, who or what created God? (And, if God answers to a higher power, to what power does that answer?)

Søren Kierkegaard, although religious himself, declared faith in God to be absurd, since it is impossible to know God, or to understand His purpose. In The Myth of Sisyphus, Camus described suicide as the most appropriate and rational reaction to the absurd — but admitted that this is not a very rewarding or worthwhile reaction.

Critics of absurdism tend to focus on two areas of the philosophy. The first is the proposition, as Camus described, that life's absence of meaning seems to remove any reason for living. Camus answers this with methods of living with the absurd: through coping or through revolt — and by pointing out that this lack of purpose presents humankind with true freedom. Others consider the theory itself to be arrogant, stating that although the purpose of life may not be apparent, that does not confirm that it does not exist.