Rawls’s theory of justice is supposed to tell us how to fairly assign the beneits and burdens of social cooperation. To answer this question one must irst develop a metric for determining whether and how much different individuals beneit from different sets of social arrangements: a metric of advantage. This metric will serve as a public basis that all members of society can appeal to for evaluating whether some people are being unfairly advantaged over others.

Rawls suggests that because it is to play this public role the metric of advantage must meet some important constraints. One constraint is that it needs to be relatively easy to evaluate how someone is doing according to the metric. For instance, Rawls rejects using simply a person’s happiness as the basis for evaluating advantage because it would be too hard to work out whether one individual was happier than another and thus there would be too much disagreement to reach public consensus on whether there was unfairness.

Another important constraint is that the metric must not be biased toward any particular conception of what makes for a good life. Different members of society have very different views about what it takes to live well but they are all going to be expected to use the same public criterion for evaluating advantage. Thus, the metric must be suitable for a “pluralistic society,” where there is disagreement about the nature of the good life. If we are to show respect for these people with their different views we need to adopt a metric that doesn’t rest on a view of the good life that some people endorse but others reject. This constraint is part of what is required to accept the use of “public reason” in political decision making, a central part of Rawls’s “political liberalism.”