IN EMILE Rousseau asserts that the sense of justice is no mere moral conception formed by the understanding alone, but a true sentiment of the heart enlightened by reason, the natural outcome of our primitive affections.' In the first part of this paper I set out a psychological construction to illustrate the way in which Rousseau's thesis might be true. In the second part I use several of the ideas elaborated in formulating this construction to consider two questions which arise in the systematic analysis of the concept of justice. These two questions are: first, to whom is the obligation of justice owed?-that is, in regard to whom must one regulate one's conduct as the principles of justice require? -and second, what accounts for men's doing what justice requires? Very briefly, the answers to these questions are as follows: to the first, the duty of justice is owed to those who are capable of a sense of justice; and to the second, if men did not do what justice requires, not only would they not regard themselves as bound by the principles of justice, but they would be incapable of feeling resentment and indignation, and they would be without ties of friendship and mutual trust. They would lack certain essential elements of humanity. Throughout, I think of a sense of justice as something which persons have. One refers to it when one says, for example, that cruel and unusual punishments offend one's sense of justice. It may be aroused or assuaged, and it is connected not only with such moral feelings as resentment and indignation but also, as I shall argue,with such natural attitudes as mutual trust and affection. The psychological construction is designed to show how the sense of justice may be viewed as the result of a certain natural develop-