In chapter 13, Mr. Arnold says the lines in question. Edgar Allan Poe is introduced to Mr. Arnold at Mrs. Whitman's home during a tea party given by Mrs. Whitman's mother, Mrs. Powers. Mr. Arnold is at the party because Mrs. Powers disapproves of her daughter's infatuation with Poe and believes Mr. Arnold should marry her daughter instead, something Mr. Arnold wants very much because Mrs. Whitman is very wealthy.
That same evening, Poe discovers Mr. Arnold is really the husband of Edmund's missing mother, Mr. Rachett. Therefore, Poe knows Mr. Arnold is being quite sincere when he tells Poe, "You have informed me much about the darker passions" (115). Mr. Arnold is sincere because he has just committed murder, kidnapping, and gold theft, all actions referred to in Poe's stories.
Poe figures out who Mr. Arnold really is the moment he sees Mr. Arnold's reaction to a story Poe relays about a man unexpectedly leaving a clothier's out of response to some unknown fear as an example of fear existing in all of us throughout the course of our daily lives:
If a man. . . if a man orders a coat, a fine coat, has it made and fitted, and yet, at the last moment chooses not to take it, might. . . might not the reason why he decides thus yield some important facts, so that what appears to be irrational becomes rational? (118)
Upon hearing this, Mr. Arnold turns red in the face and immediately leaves the party because the story fits the description of exactly what Mr. Arnold did at the clothier's the moment he saw Edmund, his stepson, walk into the store. Hence, Mr. Arnold's reaction to Poe's story helps Poe see that Mr. Arnold is really Edmund's stepfather, Mr. Rachett.