In L'Etranger, Camus uses Mersaults' experiences such as his mothers' death, killing the Arab, the trial, and his interactions with other characters throughout the novel to convey his philosophy, which satisfies all principals of existentialism.
To convey his existentialist philosophy, Camus uses the death of Mersaults' mother in the beginning of the novel. On the first page, Mersault is more concerned about the exact time of his mothers' death, and not the fact that he recently lost a loved one. This shows that Mersault feels that there is no reason to mourn for his mothers' death, and also conveys the existentialist idea that reason is powerless to deal with the depths of human life. The fact that Mersault shows no compassion ultimately conveys Camus' philosophy of existentialism. Also, at Mersaults' mothers funeral Mersault does not cry or behave the way that society expects him to act. This is because Mersault is an existentialist, and does not act in the 'appropriate' manner in which society expects, which makes him estranged from the people around him.
In the events leading up to the point when Mersault kills the Arab, the heat, sun, and light begin to affect him more and more, at which point his sensual feelings overwhelm him and cause him to pull the trigger and kill the Arab. This part of the novel shows how Mersault is estranged from nature, in the way that for the first time in the novel the sun and his sensual pleasures begin to act against him, and cause him to lose control. During the trial, Camus begins to ridicule the legal system, and make apparent the fact that Mersault is truly an outsider. Camus does this by making Mersault feel as though he is 'out of place' at his trial. He also does this by showing that Mersaults' case is rushed, due to the fact that there is a more exciting parricide case next. This reveals Camus' philosophy by Estranging Mersault from society, and legal system.
During the time in which Mersault is imprisoned, he begins to feel that he is unable to accept death and wants to 'escape the inevitable'.(p.104) This is how Camus uses Mersault to explain another principle of existentialism, which is that an existentialist begins to feel fear, anxiety and angst. The reason that Mersault feels this is that he is denied everything in prison and has nothing to deal with but himself, which makes him able to consider what is going to happen to him.
During the novel, Mersault deals with people such as his friends or acquaintances that were not readily accepted in the society of that time. When Salamano comes to talk to Mersault about his dog, instead of being compassionate and consoling the old man, Mersault tells Salamano that the pound keeps the dog for a few days until it is put down. This shows that Mersault feels no reason to lie to Salamano and tell him something to comfort him, when Mersault does not feel sorry the old man at all. This is an existentialist viewpoint in the way that Mersault has no need to conform to society how most people would.
In conclusion, Camus writes the novel in such a way that every thought and action of Mersault is used to portray his existential beliefs by showing that Mersault goes against everything that is defined as 'appropriate' in society. Camus has managed to do this well enough that one who did not have much knowledge about existentialism may gain an insight of what it is, and the beliefs that an existentialist has.