If you watch one of the interviews from The Dialogue, Learning from the Masters, (a interview series that features famous Hollywood writers), you will discover that Jim Uhls, the screenwriter of Fight Club, named the character that Edward Norton plays "Jack" for the sake of simplicity
In the movie Fight Club, Edward Norton plays the unnamed narrator, who is also referred to as Jack or "the narrator".
The narrator is a depressed insomniac who is struggling to make sense of his life. He is a white-collar worker who is dissatisfied with his job and his life, and begins attending support groups to cope with his depression. He meets Tyler Durden, a charismatic soap salesman who encourages him to rebel against society and to embrace his darker impulses. The narrator eventually starts a fight club with Tyler, and together they form a secret society of men who fight each other in order to feel alive.
The Dialogue, Learning from the Masters, (a interview series that features famous Hollywood writers), you will discover that Jim Uhls, the screenwriter of Fight Club, named the character that Edward Norton plays "Jack" for the sake of simplicity