It is clear that he is not a completely sane person, because he has a tremendous fear, the fear of ending the Usher family line. This fear occupies his entire mind and prevents him from thinking clearly. As Poe mentioned, Usher's fear causes “a mental disorder that oppresses him” (p.874) which causes him to behave strangely. He has two sides to his personality, a solemn side, when he is rational, and an exaggerated side, when fear overwhelms him. This phenomenon causes many contradictions in his words and actions, as can be seen when the narrator explains Usher's illness. "It was, [Usher] said, a constitutional evil, and for which he despaired of finding a remedy - a mere nervous affection, [Usher] immediately added, which would no doubt soon pass.”