" After your performed the exorcism, how did you know that demon wasn't still in there, hiding? How do you know it didn't go in a hibernation state, quieting down to come out later, years and years later when no one would be around to help? Hey, how do you know if the wrong spirit left? What if you expelled the person's real spirit and only the demon's spirit was there to take its place? If I believed in any of that stuff, I'd be afraid that was going to happen to me. "

― Paul Tremblay , A Head Full of Ghosts