Plato's argument for the benefits of a just life is intrinsically linked to his definition of good and its relation to people's desires. He begins by showing that when the objective of a desire is simple (e.g. quenching a thirst), the desire must be correspondingly simple. Since thirst is a simple desire, the man's objective must also be simplistic and should we assign an adjective to his objective, we would falsely complicate it. In addition, Plato believes that we would be seriously erring if we assign a value of good to an desire.
In common use, the adjective good would denote something that is good in relation to others of its kind. We consider a drink good if it contains characteristics that we look for in a drink (e.g. pleasantness or taste). Plato takes this a step further and states that something that is good must not only be good in relation to others but it must be wholly good.
Thus a drink cannot be truly good if evil results from it. This poses an interesting question for Plato's readers namely, since no one wants bad things to happen to them, why do people engage in self-destructive activities? The answer lies in the fact that the only reason that we desire to drink is that we anticipate the result of our thirst being quenched. Our appetites see no further consequences than the immediate fulfillment of our desires; they do not contemplate the results of the actions we take to fulfill our desires.
For this reason, Plato believes that we must separate the soul based on how it reacts to desires. There must be a part of the soul, Plato reasons, that contemplates the end result of our actions and makes decisions based on a higher reasoning than desire.