In this book the author argues that moral principles are principles of rational choice. According to the usual view of choice, a rational person selects what is likely to give the greatest expectation of value or utility. But in many situations, if each person chooses in this way, everyone will be worse off than need be. Instead, Professor Gauthier proposes a principle whereby choice is made on an agreed basis of co-operation, rather than according to what would give the individual the greatest expectation of value. He shows that such a principle not only ensures mutual benefit and fairness, thus satisfying the standards of morality, but also that each person may actually expect greater utility by adhering to morality, even though the choice did not have that end primarily in view. In resolving what may appear to be a paradox, the author establishes morals on the firm foundation of reason.