The concept of justice is not the same as a conception of justice. This distinction is suggested by Rawls in the early stages of his work (see, for instance, CP 47–48 and 73–74) and is stated, in a clearer way, right at the beginning of TJ. As he writes: “Those who hold different conceptions of justice can, then, still agree that institutions are just when no arbitrary distinctions are made between persons in the assigning of basic rights and when rules determine a proper balance between the competing claims to the advantages of social life” (TJ 5, emphasis added). The part of the quotation in italics is the definition of the concept of justice.

This definition of the concept is formulated in the framework of a discussion on the role of justice. Rawls starts from the idea that society is a system of cooperation for the mutual advantage of its members. He further recognizes that both identity and conflict of interests arise in this system. Principles of justice are, thus, required in order to assign benefits and burdens, or rights and duties, to the members of society. The concept of justice illustrates this point by stressing that the role of justice is to establish the most adequate equilibrium (“proper balance”) between the claims to the advantages of social life, excluding from the outset nonjustifiable discriminations between people (“no arbitrary distinctions”). However, what counts as “proper balance” and “arbitrary distinctions” is something that the concept cannot indicate and it must be substantiated by a conception of justice (such as the two principles of “justice as fairness”).