One of the main themes of The Stranger is the importance of living an authentic life in the midst of an absurd, meaningless universe, irrespective of the consequences. Meursault is very much an antihero in that he is a nihilistic murderer but at the same time someone who has exercised freedom in choosing to live the life that he leads.

Truth be told, most of us would probably want to have the courage to choose our own lives and how we live them, even if we would rightly balk at the prospect of committing the evil deed that Meursault commits.

But as Camus presents it, it's the authenticity of the life that we choose that ultimately matters. Yes, we may justly condemn Meursault's actions, but at the same time, we cannot help but acknowledge that, in true existentialist fashion, he's chosen the life that he leads rather than simply living out a particular role that society has imposed upon him. Whether we like it or not, this is what makes Meursault's whole existence authentic.

As with many existentialists, Meursault has concluded that the world is fundamentally meaningless and absurd. That being the case, he has chosen to exercise his freedom in living the kind of life he wants to lead, even though such a life has morally unacceptable consequences.

One important theme of The Stranger is life's meaninglessness, and behind this, the theme of renunciation as a path to freedom.

Meursault is a character alienated from and largely passive about everyday life. He enters into relationships with people, such as Raymond and Marie, chiefly because that other person makes it easy to do so. He enjoys sex with Marie and is passively willing to marry her, but he tells her he does not love her.

After he is tried and convicted for murder and his conviction goes to appeal, Meursault decides it makes little difference whether he dies at thirty or seventy. He reasons that we all inevitably die and that the world goes on without us. His only concern, as he tries to convince himself that his appeal will fail (which it does), is that he would feel a mad rush of pleasure at an extra few decades of life. However, he is able to quash that desire. In achieving renunciation and resignation, Meursault is then, paradoxically, able to achieve a degree of peace, knowing he is free of both expectation and the fear of death.

The absurdity of life is frequently pointed to as another important theme, and this theme emerges, as does the theme of meaninglessness, from Camus's experience of death, destruction, and ideological irrationality during World War II (as well as the absurd destructiveness of France's trying to hang onto colonial power in Algeria). This novel is one attempt to try to pick up the pieces and make sense of a world that seemed to have gone awry.

Another theme in Camus's The Stranger is the conflict between the individuality of experience and the inevitable judgements of others. Meursault's experience is not typical, but instead of being allowed to be unique, he is compared to others.

In part 1, when Meursault is at the funeral, he is aware that his actions are being judged, but he does not feel the same need to judge others. He is indifferent about his neighbor who beats his dog, and he befriends a man who justifies violence against women. There are other peculiarities about Meursault, such as his lack of reaction to his mother's death or his disinterest in typical activities like going to the cinema with his girlfriend.

In part 2, the trial serves as a review of what happened in the first part of the novel. Meursault's accounts of events are different from those of the witnesses at the trial. His justification for not visiting his mother, like his account of the incident with the man on the beach, does not make sense to the other people in the courtroom.

This impossible difference between being-for-itself and being-for-others is also an existential theme of Jean-Paul Sartre's works. Meursault, like all of us, depends on his understanding of other people to inform him about how to act in the world. His unawareness of the perspectives of others is the main problem for him during his trial. He is unprepared when his life is seen differently by others; his identity is something beyond his control, outside of his personal experience.

One of the themes of The Stranger is human alienation from oneself, each other, and from society as a whole. Meursault, the protagonist, is a symbol of this alienation because he is a Frenchman living in Algeria, a Muslim country in which he does not really fit in. He is also so alienated from himself that he does not mourn his mother's death at the beginning of the novel. The novel begins, "Maman died today. Or yesterday maybe, I don't know" (page 3). He is so alienated and distanced from his emotions that he is unable to mourn his mother's death. 

Another related theme is the meaninglessness of existence. Camus was an existentialist writer, who believed that humans must make their own meaning in a world that is essentially without meaning. The Stranger has many absurd elements to it, such as the way in which Meursault's response to the magistrate waving the crucifix about his head is to think, "I was hot and there were big flies in his office that kept landing on my face" (page 68). The magistrate tries to convince Meursault that God will forgive him for committing a murder, but Meursault finds the magistrate's reasoning absurd and meaningless because Meursault does not believe in God. Even the murder that Meursault commits is meaningless, and he does not provide any reason for killing his victim aside from his feeling pestered by the sun. His actions are without reason, as he feels as though he lives in an absurd world in which nothing is governed by reason.