Of all of Poe's stories of ratiocination (or detective stories), "The Purloined Letter" is considered his finest. This is partially due to the fact that there are no gothic elements, such as the gruesome descriptions of dead bodies, as there was in "The Murders in the Rue Morgue." But more important, this is the story that employs most effectively the principle of ratiocination; this story brilliantly illustrates the concept of the intuitive intellect at work as it solves a problem logically. Finally, more than with most of his stories, this one is told with utmost economy.
"The Purloined Letter" emphasizes several devices from "The Murders in the Rue Morgue" and adds several others. The story is divided into two parts. In the first part, Monsieur G —— , Prefect of Police in Paris, visits Dupin with a problem: A letter has been stolen and is being used to blackmail the person from whom it was stolen. The thief is known (Minister D —— ) and the method is known (substitution viewed by the victim, who dared not protest). The problem is to retrieve the letter, since the writer and the victim, as well as Minister D —— , have important posts in the government; the demands he is making are becoming dangerous politically. The Prefect has searched Minister D —— 's home thoroughly, even taking the furniture apart; he and his men have found nothing. Dupin's advice is that they thoroughly re-search the house. A month later, Monsieur G —— returns, having found nothing. This time, he says that he will pay fifty thousand francs to anyone who can obtain the letter for him. Dupin invites him to write the check; when this is done, Dupin hands the Prefect the letter without any further comment.
The second half of "The Purloined Letter" consists of Dupin's explanation, to his chronicler, of how he obtained the letter. One of his basic assumptions is an inversion of one of the aphorisms that was introduced in "The Murders in the Rue Morgue"; the case is so difficult to solve because it appears to be so simple. Beyond that, Dupin introduces the method of psychological deduction. Before he did anything else, he reviewed everything he knew about Minister D —— . Then, he reviewed what he knew about the case. With this in mind, Dupin tried to reconstruct the Minister's thinking, deciding that he would very likely have hidden the letter in plain sight. Using this theory, Dupin visited Minister D —— and found the letter in plain sight but boldly disguised. He memorized the appearance of the letter, and he left a snuffbox as an excuse to return. Having duplicated the letter, he exchanged his facsimile for the original during a prearranged diversion. Retrieving his snuff-box, he departed. His solution introduces into detective fiction the formula of "the most obvious place."
Dupin is, of course, the original eccentric but brilliant detective. He seems to be a very private person, though one with connections and acquaintances in many places. He prefers the darkness and the evening; darkness, he feels, is particularly conducive to reflection. He prefers to gather his information and to ponder thoroughly before any action is taken. He talks little; an hour or more of contemplative silence seems common. And, of course, he is an expert in the psychology of people of various types; indeed, he seems to be learned in a number of areas — mathematics and poetry, for example.
The Prefect, Monsieur G —— , is a contrast to Dupin. Whereas Dupin is primarily concerned with the psychological elements of the case, G —— is almost wholly concerned with physical details and evidence. G —— talks much and says little. Dupin considers things broadly, while G —— 's point of view is extremely narrow. Anything G —— does not understand is "odd" and not worth considering; for Dupin, that is a matter for investigation. G —— believes in a great deal of physical activity during an investigation, while Dupin believes in a maximum of thought and a minimum of physical exertion. Though Dupin says that the Paris police are excellent within their limitations, it is clear that G —— 's limitations are quite severe.
The personality of the unnamed narrator, the Dupin-chronicler, lies between these two extremes. Though he shares some of Dupin's tastes — silent contemplation in darkness, for example — and has some understanding of Dupin's methods, he seems psychologically closer to G —— than to Dupin. He seems to be a rather ordinary person with rather ordinary views and ideas. Thus, his assumptions and his interjections are often erroneous; he assumes, for example, that if the police have not been able to find the letter after their search, then it must be elsewhere. In his argument with Dupin about mathematicians, the narrator takes the common view and attitude toward mathematicians, a position that Dupin explicitly suggests is idiocy. In other words, the narrator is a mediator between Dupin and the reader. His reactions are similar to those of the reader, though he is somewhat less astute than the reader, so that the reader can feel superior to him. Naturally, such a narrator guides our attitudes toward Dupin, G —— , and the case. He is, for example, in awe of Dupin's abilities and methods; while the reader may maintain a more critical distance, he is guided in that direction to some degree. Finally, such a narrator determines the amount of information which a reader receives and guides the attention of the reader to the information received. In this case, the narrator tells us everything, but only as he receives it; because he did not witness the case being solved, the reader doesn't either.
The idea that the reader is a participant in the investigation of a crime and thus should be given all the information on which the detective bases his conclusions is quite modern. In "The Purloined Letter," the reader has little chance to participate, first because little information about Minister D —— 's character is given in the first half of the story, and, second, because there is no indication of any activity by Dupin until the second half. Poe's purpose was not to invite reader participation, but rather to emphasize rationality, stressing logical thinking as the means of solving problems. Consequently, Dupin's exposition of his thought processes are the most important part of the story. Without this highlighting of the logical investigation and solution of a problem, the detective story may never have developed; it would certainly be very different if it had. However, with this method and approach established, it became logical, and rather easy, to evolve the idea of the reader as a participant.
Attempting to determine the psychology of the criminal is an honorable tradition in detective fiction. The particular methods that are used change as more is learned about human beings, their behaviors, and their motivations; they also change, perhaps even more, as psychological theories change. Thus, much of Poe's — or Dupin's — psychology, especially the explanations, seems dated. For instance, the boy whom Dupin uses as an example arranges his face so it is as similar to the other person's expression as possible; this is supposed to give rise to thoughts and feelings that are similar to those of the other person. In the sense that outward expressions — facial expressions, clothes, and so on — are thought to influence the way a person feels, this idea is somewhat still current; however, that effect is thought to be general rather than specific, and we no longer believe that we can gain much knowledge of another person in this way. In addition, it is probably true that certain habits of thinking are likely to contribute to a person's success in a field; however, the distinctions are by no means as rigid as Poe made them seem, nor are the qualities so narrow. Although the principles that Dupin works from are rather outdated, his method is direct. This method is, of course, applicable to other kinds of problems posed in detective fiction; whenever the detective can learn and apply some knowledge of the criminal's psychology, he is closer to the solution of the crime.
Other details in "The Purloined Letter" reveal the story's era — the political system in France, Dupin's comments about poetry, mathematics, and the sciences in particular. Nevertheless, the story still reads well, and the details are overshadowed by the sweep of the puzzle and the story. Even if the story were not still interesting reading, "The Purloined Letter" would be of prime historical importance for it establishes the method of psychological deduction, the solution of the most obvious place, and the assumption that the case that seems simplest may be the most difficult to solve. Whether one is interested in good reading or has a historical interest in detective fiction, "The Purloined Letter" provides both.