The difference principle states that the only justification for a departure from strict equality in the distribution of primary goods is that the inequality in question benefits the least privileged members of society. Rawls defines primary goods as things such as "rights, liberties, opportunities, income, and wealth," of which people generally want as much as possible. This position preserves the philosophical point that inequalities are unjust while allowing for unequal distribution in practice because this new inequality helps to remedy a previous inequality.
The difference principle has a wide applicability, and can be used in areas about which Rawls did not write directly. For instance, in his 2003 article "John Rawls and Affirmative Action," Thomas Nagel points out that Rawls's philosophical position is the obvious intellectual underpinning for an affirmative action policy. It is unjust that someone should have less access to private goods because of some morally arbitrary factor such as race, class, or gender. Therefore, university admissions policies should distribute places to ameliorate such social disparities. It is worth noting, however, that Rawls also viewed a person's abilities as morally arbitrary, and subject to the difference principle. If ability is not to be used as the criterion for selection, it is difficult to see what would be a legitimate criterion.