Within the events of the story that the narrator both observes and takes part in, Roderick is the first to confess that he is struggling to remain within the realm of the rational. He tells the narrator,
I feel that the period will sooner or later arrive when I must abandon life and reason together, in some struggle with the grim phantasm, Fear.
The narrator himself begins to feel unstable when he catches his first glimpse of Madeline Usher:
I regarded her with an utter astonishment not unmingled with dread; and yet I found it impossible to account for such feelings.
He knows it is irrational to feel that way about the appearance of a woman with an obvious illness, and he can't understand or explain why she inspires such fear in him.
Furthermore, Roderick tells the narrator shortly after he arrives that he is beginning to fear the house itself; he feels that it has become sentient. Later, he confesses,
The conditions of the sentience had been here, he imagined, fulfilled in the method of collocation of these stones.
Inanimate materials such as the stone the house is built of cannot become sentient, but Roderick's imagination has broadened to the point where he thinks it is possible.
After the narrator and Roderick entomb Madeline, the narrator's grip on rationality begins to slip further, and he begins to share Roderick's delusions. He explains that
I felt creeping upon me, by slow yet certain degrees, the wild influences of his own fantastic yet impressive superstitions.
The isolation of the House of Usher, Roderick's instability, and the strain of the situation over several weeks take a toll on the narrator and leave him "aghast" as he flees the fallen house at the story's end.