In the classic Dickens story, A Christmas Carol, Ebenezer Scrooge sees a ghost. Skeptic that he is, he does not believe what he sees. When the ghost asks him why he doubts his senses, he replies: "Because, a little thing affects them. A slight disorder of the stomach makes them cheats. You may be an undigested bit of beef, a blot of mustard, a crumb of cheese, a fragment of an underdone potato."