The most general and most widely accepted principle of equality is the formal equality principle: ‘treat like cases as like’. When two persons are equal in at least one normatively relevant respect, they must be treated equally with regard to this respect (Aristotle 1984). Of course, the crucial question is which respects are normatively relevant and which are not. The postulate of formal equality demands more than just consistency with one’s subjective preferences. Formal equality is rooted in the nature of moral judgments. By definition, these moral judgments require an impartial and universalizable possible justification vis-a-vis others of the equal or unequal treatment in question—and this on the sole basis of a situation’s objective features.
A way of treating persons or, as a result of such treatment, a distribution that considers all persons as indistinguishable, thus granting them the same per capita quantity of a good, is unjust unless the persons really are alike in the relevant respect. It is only just under special circumstances: namely, if persons are equal in distributionally relevant respects, then the corresponding proportions of the distributed goods must be equal, too. Otherwise, treating unlike cases alike is unjust. Instead, justice requires a treatment of others, or a distribution, that is proportionally equal (Aristotle 1984, 1130b—1132b). That is the case when all relevant persons are treated in relation to their due. Proportional equality indicates what produces an adequate equality: if factors require unequal treatment or distribution because the persons concerned are unequal in relevant respects, then the treatment or distribution proportional to those factors is just. Unequal claims to treatment or distribution must be considered proportionally: this is the prerequisite for persons being considered equally in a just sense.