WARNING: Spoilers exist below, so if you haven't seen the movie and/or read the book and don't want to know what happens, then do not continue reading.
Then he meets Tyler Durden, a nihilist soap salesman who is disgruntled with common culture, on one of his business trips. When Jack's condo burns down, Tyler agrees to let Jack move into his dilapidated house if Jack will hit him. Jack does, and the two star a fight outside the bar. This becomes sort of ritual between the two of them, and Jack finds the insomnia goes away. Others join in the fights, and thus the fight club is born in the bar's basement. Tyler takes the club and turns it into Project Mayhem, which organizes increasingly serious anti-capitalist vandalism ventures.
During one of these missions, a fight club member dies, and Jack tries to shut down the operation in Tyler's absence, as he and Tyler have drifted apart some. He retraces Tyler's steps, and learns that fight clubs have been started in every major city. It is in one of these cities that someone calls him Tyler. Jack calls Marla and begins to realize that Tyler is a split of his own personality.
So the official Character Therapist diagnosis of Jack/Tyler is....[drum roll please]...Dissociative Identity Disorder (DID). Formerly known as Multiple Personality Disorder, DID patients have the presence of two or more distinct identities/personality states that recurrently take control of the person’s behavior. Jack had no idea he was Tyler, and there is little evidence until the end that Tyler knows he is Jack. Usually the personalities are vastly different, and no one would argue that Tyler and Jack are almost polar opposites.
DID patients can't integrate these various aspects of their identity/memory/consciousness without professional help. Each personality, or alter, has its own history, name, image. Usually, the primary identity (which is Jack) is passive, dependent, guilty, and depressed. Fits him to a "T." Jack disclosed to the doctor that he sometimes would wake up and have no idea how he got there, which accounts for the DID symptom of experiencing gaps in memory that go beyond normal forgetfulness for the primary identity (i.e, Jack "lost time").
Lastly, it is common for individuals suffering from DID to self-mutilate, be aggressive or suicidal. Clearly, Jack is all of these. He fights himself, beating himself to a pulp. He pulls the trigger of the gun "Tyler" is holding in his mouth, knowing that he is really holding a gun in his own mouth. However, this proves to be something of a fusion ritual for Jack, in that once he pulls the trigger, he shoots himself in the cheek, which doesn't kill him, but the viewer sees Tyler fall down with an exit wound in the back of his head.
Now you know all about this movie and can perhaps guess why it is a psychological favorite of mine. (What does that say about me? LOL!)
