A mystery story need not necessarily involve an intellectual theme in the ordinary sense of the term. The gradual unraveling of the mystery and the suspense created are usually sufficient to hold the reader’s interest. The reader receives pleasure from matching his wits with the character attempting to solve the mystery and the character who created the mystery. In Edgar Allan Poe’s detective and mystery stories such as “The Purloined Letter” and “The Gold-Bug,” the main characters themselves, such as Dupin and Legrand, receive this kind of pleasure, as well as expectations of monetary reward. At the same time, in their explanations of their procedures, they often make comments on human nature that serve as themes.
One such theme is expressed by Legrand as he tells the narrator how he decoded Kidd’s cipher. Legrand has the skills in logic and the past experiences with such codes to succeed at the task. Yet more fundamentally, he bases his attempt on the conviction, he says, that any mystery that one human intelligence can construct, another human can solve if the person applies his or her intellect properly and persistently. Thus armed, Legrand cracks the code with little difficulty, to the amazement of the narrator.
The experience of the narrator in trying to understand what motivates Legrand early in the story supplies a second theme. Because of Legrand’s reputation for being mentally unbalanced, at least at times, the narrator jumps to conclusions about his friend’s condition, despite the fact that the narrator is a physician. Some of Legrand’s actions are puzzling, and he takes no one into his confidence until after the discovery of the treasure. At times he seems to mystify the narrator purposely, as though he were playing a game. As a result, the narrator throughout the first half of the story has growing doubts about Legrand’s sanity. Not until the treasure is virtually in their hands does the narrator realize that Legrand has had rational purposes all along. The narrator then comments on the narrow line between sanity and insanity and how easily one can misjudge a person’s mental condition.