How is this narrator an "unreliable narrator?"

It seems that every time the speaker tells us something, he sounds crazy. The first few sentences of the story are more than enough to tell us that the narrator is an unreliable crazy person who suffers from delusions of grandeur and acute paranoia.

TRUE! --nervous --very, very dreadfully nervous I had been and am; but why will you say that I am mad? The disease had sharpened my senses --not destroyed --not dulled them. Above all was the sense of hearing acute. I heard all things in the heaven and in the earth. I heard many things in hell. How, then, am I mad? Hearken! and observe how healthily --how calmly I can tell you the whole story.