What is the theme in The Pit and the Pendulum by Edgar Allan Poe?

Poe uses the theme of insanity vs. insanity, and all the nuances in between, in many of his short stories, often charging his insane narrators with the futile task of proving that they are not mad. Often, in stories such as The Tell-Tale Heart and The Imp of the Perverse, though the respective narrators of each claim they are of sound mind and seem completely unremorseful, they are driven to confess by a persistent reminder of their crime. In other tales, such as The Cask of Amontillado, the narrator is unquestionably insane, and yet there is no remorse and no confession, and though his actions are insane, he is very levelheaded when it comes to their execution.