The answer is "He's sane."
"True - nervous - very, very dreadfully nervous had I been and am; but why will you say that I am mad? The disease had sharpened my senses not destroyed not build them. Above all was the sense of hearing acute. I heard all things in the heaven and the earth I heard many things in hell. How, then, am I mad? Harken! and observe how healthily - how calmly I can tell you the whole story."
Although he is mentally ill, he claims to be sane.
