The answer to this question can be found in the conversation that Sergeant Major Morris and the Whites have soon after he enters. As the Sergeant Major begins to tell the Whites about his experiences in India and shows them the monkey's paw, the Sergeant Major tells them about its origins and how it came to become magical:
"It had a spell put on it by an old fakir," said the sergeant major, "a very holy man."
"He wanted to show that fate ruled people's lives, and that those who interfered with it did so to their sorrow. He put a spell on it so that htree separate men could each have three wishes from it."
The Sergeant Major goes on to confirm to the Whites that he has had his three wishes, though as he says this, it is clear that the granting of wishes is something that fills him with terror and fear. He also says that he gained the paw when the first owner of the paw wished for death as his third wish. It is important to note the way in which such descriptions effectively foreshadow the action that ensues when the Whites themselves try to cheat their fate and suffer so terribly as a result.