In Edgar Allan Poe's classic short story "The Tell-Tale Heart," the narrator believes, and wants the reader to believe, that he's not mad because he so perfectly calculated and carried out every step of the murder of an old man, from the conception of the murder to the cover-up.
You should have seen how wisely I proceeded—with what caution—with what foresight—with what dissimulation I went to work!
As further proof that he's not mad, the narrator invites the reader to "Hearken! And observe how healthily—how calmly I can tell you the whole story."
To convince himself and the reader that he's not insane, the narrator explains everything he did in great detail and praises his own persistence—even his courage—in the way he sticks to his plan and never deviates from his goal of ridding himself of the old man and his "Evil Eye."
The eye of a vulture—a pale blue eye, with a film over it.
The narrator wants the reader to understand how fully and how deeply he understands the old man, how he understands the old man's fear, and how he even knows what the old man is thinking.
I knew what the old man felt ... I knew that he had been lying awake ever since the first slight noise, when he had turned in the bed. His fears had been ever since growing upon him. He had been trying to fancy them causeless, but could not. He had been saying to himself—“It is nothing but the wind in the chimney—it is only a mouse crossing the floor,” or “It is merely a cricket which has made a single chirp.” Yes, he had been trying to comfort himself with these suppositions.
The narrator commends himself with how he observes the old man's eye with "perfect distinctness" and how he directs the narrow ray of the lantern to the eye "as if by instinct, perfectly upon the damned spot."
[N]ow, I say, there came to my ears a low, dull, quick sound, such as a watch makes when enveloped in cotton. I knew that sound well, too. It was the beating of the old man's heart.
It's not madness, says the narrator, "but over-acuteness" of his sense of hearing caused by his nervousness, as he says in the opening paragraph of the story.
The disease had sharpened my senses—not destroyed—not dulled them. Above all was the sense of hearing acute. I heard all things in the heaven and in the earth. I heard many things in hell.
The narrator's obsession shifts from the old man's "Evil Eye" to the old man's loudly beating heart. When, finally, he commits the murder, the narrator is concerned much more about the old man's heart than he is about his pale blue, filmy, "vulture eye." In fact, by this point in the story, the old man's eye is little more than an afterthought.
I placed my hand upon the heart and held it there many minutes. There was no pulsation. He was stone dead. His eye would trouble me no more.
He never mentions the old man's eye again.
If still you think me mad, you will think so no longer when I describe the wise precautions I took for the concealment of the body.
Once again, the narrator praises his perfect execution of his plan.
I took for the concealment of the body. The night waned, and I worked hastily, but in silence. First of all I dismembered the corpse. I cut off the head and the arms and the legs.
I then took up three planks from the flooring of the chamber, and deposited all between the scantlings. I then replaced the boards so cleverly, so cunningly, that no human eye—not even his—could have detected any thing wrong.
When police officers arrive to investigate information from a neighbor who heard a shriek during the night, the narrator boldly takes them into the old man's room, the room where he committed the murder.
I led them, at length, to his chamber. I showed them his treasures, secure, undisturbed. In the enthusiasm of my confidence, I brought chairs into the room, and desired them here to rest from their fatigues, while I myself, in the wild audacity of my perfect triumph, placed my own seat upon the very spot beneath which reposed the corpse of the victim.
In time, the narrator's perfect plan is undone by his own nervousness and acute sense of hearing. As he sits in his chair directly above the old man's corpse, he hears the beating of the old man's heart. He first hears the heart beating as if from a distance; then, the sound grows increasingly louder until it seems to fill the room and inhabit the narrator's entire being, causing him unbearable agony.
By the end of the story, the beating heart drives the narrator into raving, raging madness—unless, of course, he was already mad when he started to tell the story.