A postulate is something that is assumed to be true; and because it’s assumed to be true, the postulate becomes the foundation for a belief or action.
In Edgar Allan Poe’s short story “The Man of the Crowd,” the narrator in the London coffee shop articulates several postulates about the people he sees in the window. He postulates that the young men he spots are junior clerks, due to their “tight coats” and “supercilious lips.” By noticing their breeches and watches, he postulates that some of the older men that he sees are senior clerks.
These distinctive postulations make the story more like a detective story because there’s a narrator studying people as if he’s trying to solve a crime or crack a case.
The detective aura of the story grows pronounced when the narrator selects one man to follow around London. In this instance, it’s like the narrator is a detective, and the man he’s tailing is a criminal that he aims to catch. This sequence derives from the narrator’s postulation. Based on the “absolute idiosyncrasy” of the old man’s face, the narrator forms a belief that this person has a singularly “wild history” that requires further investigation.
The narrator never confronts or apprehends the man. In fact, one might say that the narrator follows the man for naught. The man remains a mystery. Of course, sometimes in detective stories, the mystery is not decidedly solved.