One of Plato’s objectives in The Republic was to show that justice is worthwhile—that just action is a good in itself, and that one ought to engage in just activity even when it doesn’t seem to confer immediate advantage. Once he has completed his portrait of the most just man—the philosopher-king—he is in a position to fulfill this aim. In Book IX, Plato presents three arguments for the claim that it pays to be just:

First, by sketching a psychological portrait of the tyrant, he attempts to prove that injustice takes such a wretched toll on a man’s psyche that it could not possibly be worth it (whereas a just soul is untroubled and calm).

Next, he argues that, though each of the three main character types (money-loving, honor-loving, and truth-loving) have their own conceptions of pleasure and of the corresponding good life (each choosing his own life as the most pleasant sort), only the philosopher is in the position to judge since only he is capable of experiencing all three types of pleasure.

Finally, he tries to demonstrate that only philosophical pleasure is really pleasure at all; all other pleasure is only cessation from pain.

In all likelihood, Plato did not consider any of these to be the primary source of justice’s worth. Plato’s goal was to prove that justice is worthwhile independent of the advantages it confers, so for him to argue that the worth of justice lies in the enormous pleasure it produces is beside his point. To say that we should be just because it will make our life more pleasant, after all, is just to say that we should be just because it is to our advantage to do so. Instead, we should expect to find him arguing that the worth of justice lies in some other source, preferably having something to do with objective goodness.

This is why many philosophers, from Plato’s student Aristotle down to modern scholar Richard Kraut, believe that Plato’s real argument for the worth of justice takes place long before Book IX. They think, plausibly, that Plato locates the worth of justice in justice’s connection to the Forms, which he holds to be the most good things in the world. Justice is worthwhile, on this interpretation, not because of any advantage it confers, but because it involves grasping the Form of the Good and imitating it. The just man tries to imitate the Forms by making his own soul as orderly and harmonious as the Forms themselves.