At this point, we turn to Plato's more sophisticated treatment of the matter. In the Republic, Socrates was challenged to "tell us how justice benefits a man intrinsically, and in the same way how injustice harms him" (p. 61). To do this, he had to show what justice is. His model of the just state was that of a healthy organism, where all the parts function for the benefit of the whole, and the whole benefits the parts.
Socrates gave an elaborate account of the elements which go into the making of a city (a small state). Many different kinds of roles are undertaken by different people. The survival of the whole depends on each one performing their functions properly. Justice is sticking to one's role, doing one's own work and not interfering with others. It, along with the other virtues of a state, temperance, courage and wisdom, contributes to the excellence of that state. Indeed, justice is necessary for the other three virtues.
In the case of the individual, Plato also appealed to a model of harmonious functioning. The soul has its divisions just as the state does. There is reason, the passions and the "spirit" that enlivens them. The just man is one who keeps these in harmony with one another. "Justice, like health, depends upon the persence of a natural order governing the soul in the relation of its parts and in the conduct of the whole." This is how justice benefits a man intrisically, just as good health does.
In the discussion of Plato's theory of virtue, we found that he considered virtue to be an excellence of the soul. Insofar as the soul has several components, there will be many components of its excellence. The excellence of reason is wisdom, of the passions, attributes such as courage, and of the spirit, temperance. (Spirit is a kind of intensity of the soul, for Plato.) Finally, justice is that excellence which consists in a harmonious relation of the three parts. In the state, justice is each individual fulfilling his or her own function, without interfering with the others. So it is for the soul.
Now the question arises what relation this account of justice has to the theory of the forms. When I queried Professor Malcolm, an expert on the Republic, he replied that the account stands on its own, and so requires no reference to the forms at all. Nonetheless, there is this relation. The forms were sometimes described by Plato as ideal objects, such as triangle itself. The state and the soul that is really just is also an ideal. No actual individual attains the state of overall virtue adequate to Plato's account.