Other intuitions about fairness are simpler. Imagine you have two toys and two children, and you give both toys to one child. If the other child is old enough to speak, she will object. She might say “That’s not fair!” and she’d be correct. An even split would maximize the overall happiness of the children—give each child one toy and they’re both happy; divide them unevenly, and the child who gets nothing is miserable, her sadness outweighing the extra pleasure of the child who gets two. More to the point, it’s just wrong to establish an inequity when you don’t have to.

But fairness is more than deciding the best way to distribute the positive. We also have to determine how to allocate the negative.

Revenge—the personal form of punishment, directed against those who have wronged us personally or who have harmed  our  family or friends—has certain distinctive features. Adam Smith describes our feelings toward a man who has murdered someone we love: “Resentment would prompt us to desire, not only that he should be punished, but that he should be punished by our means, and upon account of that particular injury which he had done to us. Resentment cannot be fully gratified, unless the offender is not only made to grieve in his turn, but to grieve for that particular wrong which we have suffered from him.”