| Quote #4
And have I not told you that what you mistake for madness is but over-acuteness of the sense? (6)
Earlier, he refers to this condition as a "disease" – the implication being that it stems from his body rather than from his mind. Comments like these provoke questions about how the body and mind influence each other, and about the versions of reality of the people they belong to.
| Quote #5
[D]o you mark me well I have told you that I am nervous: so I am. (7)
We know that feeling, like the way the old nerves clang after that seventh cup of coffee. The narrator obviously doesn't think nervousness is a component of madness. It also seems separate from his "disease."
| Quote #6
They heard! – they suspected! – they knew! – they were making a mockery of my horror! (9)
It's quite possible he was right, though probably not for the reasons he gives. His creepiness probably shows through more than he knows. The fact that there are three of them suggests they were already really suspicious before they got there.