In Fight Club, Chuck Palahniuk has “Jack” find old copies of Reader’s Digest and thus introduces the motif which begins with “I am Joe’s Prostate." How, and to what affect, does the text use “I am Joe’s XXXX” to reveal and convey information to the reader.
The use by Chuck Palahniuk of the borrowed concept from Reader's Digest, in which lessons on human anatomy and health care were conveyed through a faux first-person discussion of various body parts, is part of an intriguing transformation of a story that is more psychological then originally suggested early in the story. As it becomes more apparent that something is "off" in the story, the use of the old Reader's Digest routine takes on more and more significance.
Fight Club takes place in the Narrator's mind. An insomniac seemingly trapped in a mundane existence, he finds comfort in support groups, the attendance in which requires him to fake medical conditions he does not have. Tyler Durden also leads a seemingly mundane existence, but introduces the Narrator to a new and very different world, that of fighting as a means of existing. Fighting becomes existential. Tyler also "schools" the Narrator in how to live a more substantive life, albeit one centered on civil disobedience, in effect, a method of enhancing the meaning of one's life, rather than continuing as a faceless drone in another company.
As Tyler and the Narrator grow closer, including living together following the explosion in the latter's apartment, they begin to blur into a single individual. As it becomes clear that Tyler is a figment of the Narrator's imagination, the significance of the Reader's Digest "I am Joe's prostrate" feature becomes apparent. The references to body parts and emotions is meant to convey the blurring of the distinction between the Tyler and the Narrator, with "Joe" filling in for the Narrator. As the Narrator repeats phrases like "I am Joe's boiling point" and "I am Joe's enraged sense of rejection," and "I am Joe's complete lack of surprise," it suggests more and more that Tyler serves as the complete embodiment of what the Narrator views as a more ideal self.