One of Edgar Allan Poe's most popular short stories is "The Tell-Tale Heart." The question of the narrator's sanity is not unusual especially because he begins his tale by insisting that he is not crazy!
TRUE!—NERVOUS—VERY, very dreadfully nervous I had been and am; but why will you say that I am mad? The disease had sharpened my senses—not destroyed—not dulled them.
The narrator insists he is not mad, but admits that he suffers from the disease of madness. His argument is that rather than being incapacitated in any way, his condition has improved his senses. This conclusion is based upon his insanity for logically one cannot be ill and be better off for it. However, he continues to insist that he is not crazy throughout the story.
The one paragraph that seems to solidify not only his insanity but also his guilt is found below:
It is impossible to say how first the idea entered my brain; but once conceived, it haunted me day and night. Object there was none. Passion there was none. I loved the old man. He had never wronged me. He had never given me insult. For his gold I had no desire. I think it was his eye! yes, it was this! He had the eye of a vulture—a pale blue eye, with a film over it. Whenever it fell upon me, my blood ran cold; and so by degrees—very gradually—I made up my mind to take the life of the old man, and thus rid myself of the eye forever.
Note first that "the idea" has taken ahold of his mind: it haunts him continually. This is clearly obsession in the strictest sense. The "haunting" not only infers that the idea is never out of his mind, but also that he has no control of the idea. This supports the conclusion that the narrator is mad.
Additionally, he recognizes that he has no reason to wish the man ill. He states that he is not driven by passion (anger); there is no desired outcome that he anticipates from the man's death. In fact, he claims that he "loved the old man." He does not harbor any ill feelings toward him, for the old man never harmed him or insulted him. And the speaker does not want the old man's money either. With no valid reasoning present, these statements further convince the reader of the instability of the narrator's mind.
What very neatly supports his insanity is the younger man's obsession with the old man's eye. In his madness, he has fixated on the elderly man's blind, blue eye.
I think it was his eye! yes, it was this! He had the eye of a vulture—a pale blue eye, with a film over it.
The narrator's paranoia is obvious. He personifies the eye, treating it as if it has a life of its own, apart from the will of the old man. He irrationally believes that the eye is the cause of his distress and only in killing the man can he be rid of it. When entering the man's room in the dead of night, the speaker believes that the eye follows him, but the film over the eye makes this assumption irrational and impossible.
Having established that the man telling the tale is insane, consider if the man is culpable (or accountable) for committing the murder. Could he not be found innocent by reason of insanity?
The insanity plea in a court of law is only useful if it can be established that the madness present prevented the perpetrator of the crime from distinguishing between right and wrong. More specifically put, the definitive measure of insanity was based upon the trial of Daniel M'Naghten in England in 1843. M'Naghten murdered the Prime Minister's secretary because he believed the Prime Minister was "conspiring against him." Findings in this case became the standard measure for insanity, and are still used in U.S. courts:
The "M'Naghten rule"...created a presumption of sanity, unless the defense proved "at the time of committing the act, the accused was laboring under such a defect of reason, from disease of the mind, as not to know the nature and quality of the act he was doing or, if he did know it, that he did not know what he was doing was wrong."
The narrator did know that what he was doing was wrong, and fully comprehended exactly what he was doing.
Certainly the murder is premeditated. The man announces to the reader that once he ascertained (erroneously) that the eye was the source of his difficulties, he immediately decided that he had to kill the man to be free from its non-functioning gaze. There also can be no doubt that the narrator knew the difference between right and wrong—knew that he was committing a crime.
First of all, he does not want the old man to cry out and alert the neighbors—who would, ostensibly, call the police. Then he hides the body. (Actually, he literally crows with delight while recounting how brilliantly he disposed of the evidence of his deed.) He buries all beneath the floorboards in an attempt to conceal what he has done.
When the police eventually arrive, he pretends that all is well. He invites them in to search the house. He explains that the old man is in the country. He even takes the men into the old man's room and with insane arrogance invites them to sit, rest and chat even while the body parts rest squarely beneath their feet.
Here again his madness is glaringly apparent in that soon thereafter he is convinced that he hears the beating of the dead man's heart. (He is delusional). He soon becomes excessively agitated, while also noticing that the policemen seem oblivious to his discomfort while the sound grows ever louder in his ears. Ultimately, with paranoia bursting from within him, he is certain that the men are well aware of what he is done and they are "making a mockery of [his] horror."
All at once, he begins to rave in his madness:
I felt that I must scream or die! and now—again!—hark! louder! louder! louder! louder!
“Villains!” I shrieked, “dissemble no more! I admit the deed!—tear up the planks! here, here!—It is the beating of his hideous heart!”
The narrator is not only insane, but he is also guilty of murder.
Yes he is mentally insane and he does feel guilty because the heart he hears is really his own heart from the guilt of killing the old man with the cataract (the creepy eye) even though he is convinced its the dead mans under the floor boards heart.