In the movie "Fight Club," there are many themes that are portrayed throughout the movie. There is one major theme that prevails among all of these. It portrays the duality of man. Dualism is defined as both a doctrine that the universe is under the dominion of two opposing principles one of which is good and the other evil, and also, a view of human beings as constituted of two irreducible elements (as matter and spirit). There are many examples of this during the entire movie. The Narrator lets the viewer into his mind by telling them some of his random thoughts throughout the movie. The storyline of the movie must be known to understand the themes in the movie.
The Narrator, whose name is never learned, but he goes by Jack, tells the storyline. Jack has insomnia and he starts going to support groups and that helps him to sleep. He meets Marla, another person at the support groups that uses them to overcome her own misfortunes. Jack returns from a business trip to find that his condo and all his belongs have been blown up, and he is left homeless. He moves into an old run down house with a friend that he meets on business trip (Tyler). He and Tyler begin a Fight Club behind a bar, which eventually turns into a sort of secret society in the basement of that bar. Tyler and Jack send out the men who belong to this society on missions, finally forming a large army. The society spreads to other large cities and becomes bigger and more devastating with each new mission. Jack comes to realize that Tyler is his alter ego, and that he must be stopped along with the others in "Project Mayhem". Jack must triumph over this alter ego by getting rid of Tyler. .
The members of the Fight club are leading a double life. Jack narrates, "Who you were in fight club is not who you were in the rest of the world. You weren't alive anywhere like you were alive at fight club. But fight club only exists in the hours between when fight club starts and when fight club ends.