In Shakespeare's play, Macbeth says:
Present fears are less than horrible imaginings.
This is an important truth. We will often find in our lives that things we dread turn out to be less terrible if and when we face them. This even includes final exams. The imagination can make the unknown seem worse than almost anything that is real.
The author of "The Monkey's Paw," W. W. Jacobs, displays brilliance in the way he evokes terror in the reader's mind without showing anything at all. The reader knows that Herbert was horribly mangled in the machinery at the textile plant. His father had to identify the body. When his wife is insisting on his wishing for Herbert to return to them:
The old man turned and regarded her, and his voice shook. "He has been dead ten days, and besides he—I would not tell you else, but—I could only recognize him by his clothing. If he was too terrible for you to see then, how now?"
So the reader must imagine a man who was so mangled that his father could only identify him by his clothing. And further, the reader must imagine the effects on the body of decaying for ten days in a grave. This is a case where the reader's horrible imaginings are worse than the sight of Herbert himself. Yet the author describes nothing but a closed and bolted door with someone knocking on the outside.
The scene is beautifully handled. Mr. White comes downstairs looking for a candle.
At the foot of the stairs the match went out, and he paused to strike another; and at the same moment a knock, so quiet and stealthy as to be scarcely audible, sounded on the front door.
Why would there be such a stealthy knocking? It makes the incident seem worse than if there had been a booming noise. The fact that the knocking is "stealthy" and "scarcely audible," for whatever reason, makes the reader imagine that Herbert is there and that he knows he looks so horrible that he hardly dares seek admission to his old home. This stealthy knocking is intended to make the reader feel sure it must be Herbert outside. And as the knocking becomes more and more insistent, the reader, along with the parents, is certain it must be Herbert. Why? Because Herbert is dead but caught between two worlds, the world of the living and the world of the dead. He must get in somewhere. And his parents owe that to him. If the house was paid for by his accident, then the house is really his. Furthermore, if he has come in response to the wish made with the monkey's paw, all the supernatural powers of the cosmos are compelling him to obey that wish.