The last chapter explored some difficulties facing the quest for rational justification in political life. This chapter and the next take up Plato's influential effort to overcome them, and explore some variants of the general justificatory strategy that he pioneered. I will call theories that follow this strategy common-good arguments. These arguments form a very broad church and have come in many shapes and sizes. Despite these many differences, however, they share a distinguishing feature. They all assume that the value of political arrangements and forms of collective organization, along with the beliefs about justice and other ethical ideals that hold them in place, must ultimately be explained in terms of their contribution to the well-being and happiness of everyone living within them. Insofar as they meet this condition, these arrangements, practices, and beliefs are part of a common good, or so theorists in this tradition maintain.

This chapter explores Plato's contention that, properly understood, justice is part of the common good in this sense, something that benefits everyone. As we shall see, Plato's seminal proposal launched a distinctively perfectionist conception of political life, one later refined by his pupil Aristotle and still influential today. Having (in this chapter) described the contours of this classical perfectionist account of the common good, in the next we shall consider the most influential modern variant of the common-good approach, utilitarianism. That chapter ends with a critical discussion of some of the problems facing this approach as a whole.