We should choose friends who are free from strong desires as much as possible. Vices spread like diseases. They pass from one person to their neighbor and harm anyone who gets too close. During a plague, we're careful not to sit near infected people whose disease is active. We know we'll catch it from their breath if we do. In the same way, when choosing friends, we must pick people who are as clean from corruption as possible. The surest way to get sick is to mix what's healthy with what's rotten.
We should choose for our friends men who are, as far as possible, free from strong desires: for vices are contagious, and pass from a man to his neighbour, and injure those who touch them. As, therefore, in times of pestilence we have to be careful not to sit near people who are infected and in whom the disease is raging, because by so doing, we shall run into danger and catch the plague from their very breath; so, too, in choosing our friends' dispositions, we must take care to select those who are as far as may be unspotted by the world; for the way to breed disease is to mix what is sound with what is rotten.