Support your answer with textual evidence.
This sort of story is Poe's specialty. Personally I think Poe is interested in madness and the extent of their (characters') own nightmares or fantasies. Poe writes that Usher "entered, at some length, into what he conceived to be the nature of his malady." What exactly is his "malady" we never learn. Even Usher seems uncertain, contradictory in his description: "It was, he said, a constitutional and a family evil, and one for which he despaired to find a remedy--a mere nervous affection, he immediately added, which would undoubtedly soon pass off." The Narrator notes an "incoherence" and "inconsistency" in his old friend, but he offers little by way of scientific explanation of the condition. As a result, the line between sanity and insanity becomes blurred, which paves the way for the Narrator's own descent into madness.
