To begin our understanding of social morality, we must see why it is necessary: why human social life confronts us with situations in which each individual relying on her own reasoning about her ends can lead to results that are bad for all. This chapter considers instrumental reasoning in “mixed motive games” in which although all benefit from cooperation, all are tempted to cheat, but if they do so, all will be worse off. In the history of social and moral philosophy, those who have taken this problem most seriously, and from whom we have the most to learn, are Hobbes and his contemporary followers such as David Gauthier. Rational individuals, they argue with great sophistication, can reason themselves out of such situations by agreeing to social morality; social morality is a tool of rational individuals who are seeking to promote their values and ends.