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. So we see two distinct parts of the soul. The first is said to be appetite (which desires without reason) and reason (which considers the consequences). Reason may thus work against anything that is not for the total good of the man. Plato holds that if the desire were truly for a good drink, reason would never oppose it. Our usage of the word good, however, has come to denote an expectation of usefulness to our purpose; although this may be relative to the end result that we experience from the object. For example, we call a knife good because it is sharp and cuts well but if the end result is that we cut ourselves, we would say that the knife would have been better if it were not so sharp. We need to consider everything that is relevant to the action or object and determine its possible consequences before we denote it as good.
Once we have done this, and assigned a value to each object or action, then Plato believes that we can say that "everyone wants the things that really are good" even if the person does not realize the true nature of what is good (505d). This Plato calls what we want' and it does not necessarily coincide with what we think is good. In light of this difference, Plato says that a tyrannical soul will be least likely to do what it wants' (577d). Can we then say of Leontius (439e) that he perceived himself as doing something good or forwarding his happiness? Plato more represents him as a man overpowered by a tyrannical desire, led to do something that he both disapproves of and is contrary to his interests. According to Plato, if Leontius were freed of his desires, he would wish (as the tyrannical man would) that he was acting otherwise.
Plato states his views on this overpowering desire. by referring to the division of the soul. All desires (whether a product of the appetitive, or the desire for honor which stems from the spirit, or the desire for knowledge which comes from reason) are for particular goals or objectives (e.g. drink, honor and knowledge) (580d). These objectives may be either good or bad for it is not as good that we desire them. Rather we desire them as drink, honor and knowledge. This forms the base for Plato's argument that the unregulated life is...