The vast majority of Kesey's novel takes place in a mental institution, of course, and for some characters, that seems an appropriate place for them, as many have serious mental issues. But mental health care in general and in Nurse Ratched's ward in particular, electroshock therapy, heavy sedation, lobotomies and control through fear and humiliation are the order of the day, and it doesn't take long before the reader sides with McMurphy and company. Such was typical of institutions in the 1950s when the story takes place.
So ask yourself the question, how does society define what is sane and insane? And was it sane to treat the patients to such harsh and demeaning conditions? What if McMurphy is sane (OK, and a little eccentric, but sane)?
At the end of the story we find that Chief has been sane all along, perhaps the most sane observer in the entire place, and so Kesey perfectly positions us to question our thinking about sanity.