“Now imagine that the human body is a gigantic ocean liner, and that all of your brain cells are the crew. Now one of these crew cells is up on the bridge. He’s the captain. But he never knows precisely what the rest of the crew belowdecks is doing; all he knows is that the ship keeps running smoothly and the job’s getting done. Now the captain is you—it’s your waking consciousness. And what happens in dual personality—maybe—is that one of those crew cells down belowdecks comes up on the bridge and takes over command of the ship. In other words, mutiny. Does that help you understand it?”

Chris was staring in unblinking incredulity. “Father, that’s so far out of sight that I think it’s almost easier to believe in the flipping Devil!”

The Exorcist
William Peter Blatty