A good man—even a good American—running to catch a train on an important assignment has to pass by the beating of a dog on the street and concentrate on catching the train; and, once on the train, he has to consider the assignment about which he must do something, rather than the dog-beating about which he can do nothing. If he is running fast enough and his assignment is mortally important, he will not even notice the dog-beating when he passes it by.
