I was astonished, too, at the vast extent of his reading — and above all I felt my soul enkindled within me by the wild fervor, and what I could only term the vivid freshness, of his imagination.

The narrator here describes his feelings upon meeting Dupin for the first time. Although it’s easy to consider the narrator as merely a convenient blank slate or just Dupin’s intellectual foil, his instant attraction to Dupin’s mind suggests that he, too, is a bit eccentric and enjoys unusual people and experiences. He likes the “freshness” and novelty of Dupin’s mind, which a more traditional or adventure-adverse person certainly would not. This hint of eccentricity in the narrator is confirmed when he and Dupin take up their unusual lifestyle, full of nighttime walks and “wild whims.”

I shuddered as Dupin asked me the question. “A madman,” I said, “has done this deed — some raving maniac, escaped from a neighboring Maison de Santé.”

The narrator has this answer for Dupin when he presents all the known facts of the murderer to the narrator in the form of a logic puzzle. When given all the details, the narrator can only imagine they add up to a madman as the prime suspect. This unsurprising assessment demonstrates that while the narrator admires and respects Dupin’s analytical abilities, he himself lacks these faculties. In fact, stating that only a madman could have committed a brutal murder is an extremely common, generic response. The narrator thus possesses an ordinary mind, used to highlight Dupin’s extraordinary one.