| Quote #1
It is impossible to say how first the idea entered my brain; but once conceived, it haunted me day and night. (2)
Obsession forces the narrator's version of reality into a narrow tube. If the narrator was thinking about inventing a cure for cancer or something, this tunnel vision might be a good thing. Here, his version of reality is dangerous to himself and others.
| Quote #2
[F]or it was not the old man who vexed me, but his Evil Eye. (3)
The narrator attributes almost godlike power to the eye, as indicated by the capital letters. Perhaps he thinks the old man is inhabited by a horrible spirit.
| Quote #3
And it was the mournful influence of the unperceived shadow that caused him to feel – although he neither saw nor heard – to feel the presence of my head within the room. (5)
This is an interesting passage because the narrator speaks for the old man. Throughout the eighth night the narrator imagines the old man's feelings, using his own experience as kind of a template, in a fit of pre-murder empathy.