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 Although the principles that Dupin works from are rather outdated, his method is direct.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.
