The theme conveyed in the excerpt is that the insane live in a reality of their own due to the narrator's distorted perception and obsessive thoughts.
The narrator's descent into madness is evident through his delusions and justification of his actions, such as believing he hears things from heaven, earth, and hell. His distorted perception of reality and obsession with the old man's eye demonstrate the theme of living in a self-created, irrational world.
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