 Formal and proportional equality is simply a conceptual schema. It needs to be made precise – i.e., its open variables need to be filled out. The formal postulate remains empty as long as it is unclear when, or through what features, two or more persons or cases should be considered equal. All debates over the proper conception of justice – over who is due what – can be understood as controversies over the question of which cases are equal and which unequal (Aristotle, Politics, 1282b 22). For this reason, equality theorists are correct in stressing that the claim that persons are owed equality becomes informative only when one is told what kind of equality they are owed (Nagel 1979; Rae 1981; Sen 1992, p. 13). Every normative theory implies a certain notion of equality. In order to outline their position, egalitarians must thus take account of a specific (egalitarian) conception of equality. To do so, they need to identify substantive principles of equality, which are discussed below.