“It is a mistake to think that some of our impulses—say mother love or patriotism—are good, and others, like sex or the fighting instinct, are bad. . . . There are situations in which it is the duty of a married man to encourage his sexual impulse and of a soldier to encourage the fighting instinct. There are also occasions on which a mother’s love for her own children or a man’s love for his own country have to be suppressed or they will lead to unfairness towards other people’s children or countries.”