The idea that fills the inhabitants of the house is that there is never enough money.
For the mother money is an end in itself; by contrast, Paul hopes that enough money will finally satisfy his mother and she will love him. But like many who desire material possessions, the mother never seems satisfied, nor does she repay Paul with the love he desires.
In the exposition of the story, the lines read like those of a fairy tale:
There were a boy and two little girls. They lived in a pleasant house, with a garden, and they had discreet servants, and felt themselves superior to anyone in the neighborhood.
What's more, the power to predict which horse will win a race and bring them money seems magical. Nevertheless, life is empty and false because the mother becomes engaged in a futile quest for the satisfaction of being rich, and she neglects her children. Indeed, at the center of her heart "was a hard little place that could not feel love." When Paul becomes ill, his mother feels that her heart has gone, "turned actually into a stone," and after Paul's death, her brother upbraids her, declaring that the poor unloved boy is "best gone out of a life where he rides his rocking horse to find a winner" for his ruthless family.