This novel presents the reader with, at the beginning at least, a clear line that is drawn between those who are "sane" and those who are "insane." However, what happens as the novel progresses, especially with the introduction of McMurphy into the asylum, is that these supposedly secure and discrete categories become gradually more and more confused and blurred so that terms such as "sane" and "insane" lose their meaning and it becomes very difficult to determine what importance these terms actually have any more. A classic example of this is when McMurphy leads the patients in watching the TV even though Nurse Ratched has turned it off:
And we're all sitting there lined up in front of that blanked-out TV set, watching the gray screen just like we could see the baseball game clear as day, and she's ranting and screaming behind us. If somebody'd of come in and took a look, men watching a blank TV, a fifty-year-old woman hollering and squealing at the back of their heads about discipline and order and recriminations, they'd of thought the whole bunch was crazy as loons.
Note how Nurse Ratched, who is of course supposedly the "sane" character, looking after the "insane" patients, is shown to be anything but sane. Likewise there is a sense in which there is a curious logic to the rebellion of McMurphy and the other patients that suggests they are not insane at all. The novel therefore profoundly questions the usefulness of such notions as sane and insane and exposes a more messy, blurred version of reality where the various problems that people have are shown to be, at least in part, created by society and the way they are treated. Chief Brom is of course a case in point, and the reader ends the novel with a very different idea of what mental illness constitutes. This is the version of reality that the novel presents the reader with, which challenges their previous notion of "reality" through asking some deep questions about the nature of mental illness.