The narrator begins the story as if he is writing an essay on the subject of what Poe often called ratiocination, which means something like logical analysis and solution of problems. Poe's purpose seems to be to establish a strong contrast between the opening and the story he tells ostensibly as a mere illustration of the principle he has been discussing in his essay. The reader is beguiled into thinking that this is just an philosophical essay, an intellectual exercise; but then Poe slowly leads the reader into the scene of the crime, and the story gradually becomes more and more emotionally moving until the actual crime is described in detail. The actual murders are, of course, the high point of the story, but Poe designs his story in such a way that the horrific scene is not described until near the end. (Truman Capote did something similar to this in his true-crime novel In Cold Blood.) From a discussion of things like whist, chess and checkers, the narrative has led the reader into a nightmarish scene in which an orangutan from Borneo in what is now Indonesia is attacking two screaming women with a straightedge razor. In this way the horror of the actual bloody massacre is enhanced by contrast with the calm discussion of the process of logical thinking.
The opening sentence reads:
THE MENTAL FEATURES discoursed of as the analytical, are, in themselves, but little susceptible of analysis.
Then after a lengthy discussion of analysis, or ratiocination, the narrator writes:
The narrative which follows will appear to the reader somewhat in the light of a commentary upon the propositions just advanced.
So the subject is ostensibly analysis, including a comparison of chess and checkers as well as the card game of whist; while the story of the murders in the Rue Morgue is only "a commentary upon the propositions just advanced." In addition to creating a strong contrast between cold reason and savage emotion, Poe is introducing C. Auguste Dupin, a very unusual character who became Arthur Conan Doyle's inspiration for his world-famous character Sherlock Holmes.