Personally, I thought the movie was suppose to be a different commentary on mental illness. So, I think one of the themes that the movie tries to touch on is that those who we perceive to be mentally insane might not actually be. That perhaps their "insanity" is really a deeper understanding of something than we sane people have. Really, how do we decide if something is to be considered sane or insane? As far as I can see, it is done by consensus. Nearly 99% of the population of the world believes that there is an all knowing all seeing higher power that is responsible for creation. We do not label these individuals as insane but we do label the person who sees monsters in their refrigator insane. The airplane engine that was suppose to kill Donnie actually came from the future. He avoided his death the first time due to his sleep walking/schizophrenia which creates the parallel universe where the engine falls from the airplane. We would call someone who avoided such a death as lucky but luck had nothing to do with it. Donnie avoided it because of his mental "illness".