While the first two answers portray the Whites as making the first wish in “The Monkey’s Paw” as a joke, I disagree. The Whites are driven by greed and ambition, just as most people are. It may be that they do not fully believe that the paw will grant their wish, but they cannot prevent themselves from wishing. It is sort of like buying a lottery ticket knowing that you will almost surely not win, but being unable to forego the chance.
To show that this is the case, we can see at least two pieces of evidence from the story. First, when Sergeant Major Morris throws the paw on the fire, Mr. White cries out in astonishment and grabs it out of the fire. You do not reach into a fire to rescue something that you think is a joke. Second, Mr. White forces money on Morris in exchange for the paw. If you really think something is a joke, you are not going to force someone to take money for it when they are already ready to burn it.
In whatever spirit the first wish is made, it is for two hundred pounds. Herbert White suggests this particular amount because it would be enough to pay off the mortgage on their house. The consequence of this wish is that the parents get the money, but that Herbert dies. He is killed in an accident when he is “trapped in the machinery” at his workplace. A man from the workplace comes and gives them the money as compensation.
The next wish stems from the first. Ten days later, Mrs. White realizes that they have two wishes left and could use one to bring Herbert back from the dead. She pushes her husband until he gives in. As the story tells us, “He raised his hand. ‘I wish my son alive again.’” The consequence of this is that Herbert’s corpse is (apparently) reanimated. All we know for sure is that some hours later something starts knocking on the Whites’ door in the middle of the night. We never “see” what is knocking, but the Whites are sure it is Herbert. Mrs. White thinks Herbert will be normal again, but Mr. White does not. He remembers that Herbert was mangled beyond recognition in the accident and he knows the corpse has been rotting for 10 days. He is sure that the knocking is coming from Herbert’s corpse, and not from a Herbert who has been brought back to life as he was before the accident.
Just as we are not told for sure what is knocking on the door, we do not know for certain what the third wish is. Mrs. White has been getting a chair so she can open a bolt on the door that is too high for her to reach. Mr. White is frantically looking for the paw because he does not want “the thing outside” to get in.
He heard the creaking of the bolt as it was slowly opened, and at the same moment he found the monkey’s paw and frantically breathed his third and last wish. The knocking stopped suddenly, though it still echoed in the house.
When Mrs. White opens the door, there is no one there and the road is quiet and empty. We can assume that Mr. White wished for his son to return to the dead and that the consequence of his wish was that Herbert did so.