What does the narrator say in the first paragraph that might lead the reader to think he is insane?

In the first paragraph, the narrator says a few things that lead us to question his mental stability.
First, he admits that he has a "disease" but that it has sharpened his senses. He proclaims that his mental acuity is not dimmed on account of this malady. The narrator's insistence that his disease has not affected him negatively inspires us to harbor skepticism about his veracity. After all, how can physical or mental debilitation or weakness sharpen one's senses? What the narrator says doesn't make any sense.
Next, he claims that his condition has strengthened his sense of hearing. He insists that he can now hear everything that goes on in heaven and on earth. The narrator also maintains that he has heard many things in hell. His preposterous claims lead us to question his sanity.
Lastly, he claims that his calm retelling of a still-obscure story (which will commence immediately) is proof of his health and sanity. His reasoning is flawed, however: we cannot conclude that an individual is sane just because they tell a story calmly.