In a famous review of Nathaniel Hawthorne's Twice-Told Tales, Poe defined the short story as follows:
A skilful literary artist has constructed a tale. If wise, he had not fashioned his thoughts to accommodate his incidents; but having conceived, with deliberate care, a certain unique or single effect to be wrought out, he then invents such incidents--he then combines such events as may best aid him in establishing this preconceived effect. If his very initial sentence tend not to the outbringing of this effect, then he has failed in his first step. In the whole composition there should be no word written, of which the tendency, direct or indirect, is not to the one pre-established design. And by such means, with such care and skill, a picture is at length painted which leaves in the mind of him who contemplates it with a kindred art, a sense of the fullest satisfaction. The idea of the tale has been presented unblemished, because undisturbed; and this is an end unattainable by the novel. Undue brevity is just as exceptionable here as in the poem; but undue length is yet more to be avoided.
Poe expected the reader of "The Cask of Amontillado" to make many deductions from the information his narrator Montresor provides. This is something new in fiction writing. It seems to foreshadow Ernest Hemingway's "iceberg theory." There is a great deal hidden beneath the surface in Poe's masterpiece. Much of this is deducible to "the mind of him who contemplates it with a kindred art."
For example, Montresor twice tells Fortunato, "I have my doubts." For one thing, this suggests that he is anxious to buy more of the Amontillado at the bargain price he mentions but that he has to be assured it is genuine. He has already bought and paid for the pipe of wine, i.e., 126 gallons. Why does he need to satisfy his doubts after the fact? The obvious deduction from his speech and body language is that he wants to buy more of the wine before word gets around that a shipment is being offered at a bargain price. That is why he pretends he is going to see Luchesi, since he pretends he was unable to find Fortunato.
Furthermore, by telling Fortunato he has his doubts about the wine, he is providing his friendly enemy with the notion that it would be easy to cheat him, to discourage him from buying more of it. Montresor has suffered a "thousand injuries" at the hands of Fortunato and knows him very well. He knows that if the wine really existed, Fortunato would sample it, lick his lips, frown, shake his head, and finally declare that it is only ordinary sherry. Then, assuming it was genuine Amontillado, this connoisseur would rush off to find the seller. It would have to be someone aboard a newly arrived ship from Barcelona, which would be easy for a man with Fortunato's experience to do. Fortunato is a rich man. He could buy up the entire cargo and make a fortune bottling and reselling it. When Montresor found out about it, he would laugh and call it an excellent jest.
There is another reason which the person who contemplates the story with a kindred art should be able to deduce. If for some reason Fortunato cannot accompany Montresor to his palazzo that night, Fortunato will still want to taste this gourmet sherry at the earliest opportunity. Since the Amontillado does not really exist, as we ourselves realize later on, Montresor will have to bring Fortunato a bottle of ordinary sherry and claim that this came from the pipe--which means that he will have to concoct a whole new way of killing his hated enemy with impunity. Fortunato will taste the sample and know it is not genuine. He will assume Montresor has been cheated and will lose all further interest. Fortunato would undoubtedly learn that there was no Spanish ship in the harbor with a cargo of sherry, but Montresor could claim that the cask was delivered to him and he never saw where it came from. This supposed fact could partially explain why he has his doubts.
The statement "I have my doubts" is just one of dozens of clues to understanding the story, particularly to understanding how Montresor planned and executed the perfect crime he confesses to in his narrative. Every detail that raises questions in the reader's mind can be explained, not merely as plausible, but as essential to the author's creation of "a certain unique or single effect." The words "single effect" define the modern short story. Poe means a feeling, an emotional effect. "The Cask of Amontillado," with its chilling ending, is a perfect illustration of Poe's rationale.