The Tripartite Theory
In the Republic, Plato introduces a new understanding of the human soul: the Tripartite Theory of the Soul.
The soul has three parts: "reason, "spirit," and "appetite." All parts of the soul have desires, but desire in appetitive and spirited parts is not a matter of belief about the good. The desires in these parts arise independently of any beliefs about what is good or bad. In the appetitive part, desires form in reaction to events in the body. In the spirited part, desires form in terms of the habitual responses developed in upbringing.
In the Tripartite Theory of the Soul, Plato abandons the Socratic intellectualist theory Socrates advocates in the Protagoras.
"There is reason to believe that Socrates thought that there is no such thing as acting against one's own better judgment. What does happen is that reason in certain circumstances gets confused and, instead of holding on to its better judgment, follows some other judgment. If reason knew the truth, it could never get confused in this way. Thus, according to Socrates, such cases reveal nothing but a failure of reason which in its weakness does not hold on to the true belief, but accepts a false one and acts on it. Plato, Aristotle, and their followers, on the other hand, believed that such cases could not be explained as purely intellectual failures, that one had to assume that besides reason there is an irrational part of the soul with its own needs and demands which may conflict with the demands of reason and which may move us to act against the dictates of reason, if reason has not managed to bring the irrational part of the soul firmly under its control." (Michael Frede, "The Stoic Doctrine of the Affections of the Soul," 96.)
Sometimes a person feels thirsty but decides not to drink. How is this possible?
According to the Tripartite Theory, there are two desires in play. One stems from appetite. It arises naturally in reaction to events in the body, feeling thirsty. In the absence of a desire from reason, this appetitive desire would motivate the person to drink. Reason, however, in the example, has the belief that in the circumstance drinking is not what should be done. If the parts have the proper organization, the normal connection between the appetitive desire and action is interrupted. Reason overrides appetite.
"Is there something in the soul of those who are thirsty but refuse to drink, something bidding them to drink and something different, something forbidding them, that overrides the thing that bids them to drink? And doesn't the thing that forbids in such cases come into play, if it comes into play, as a result of calculation, while what drives and drags them to drink is a result of feelings and diseases? Hence isn't it right for us to claim that they are two, and different from one another? We'll call the part of the soul with which it reasons the λογιστικὸν and the part with which it lusts, hungers, thirsts, and gets excited by other appetites without reason and appetitive part, companion of indulgences and pleasures." (Republic IV.439c-d)
Reason Rules in a Harmonious Organization
There are different possible organizations among the parts of the soul. One organizations constitutes proper psychological functioning. When the parts are so organized, they are in "harmony." Reason rules, spirit is reason's ally, and appetite is held in line. Since reason knows the good, a human being with a properly functioning soul acts for the sake of the good.
- rationality is proper psychological functioning
- proper psychological functioning is the harmonious organization of the three parts of the soul
- a human being whose soul is in harmony acts for the sake of the good
"It is appropriate that the reasoning part should rule, since it is really wise and exercises foresight on behalf of the whole soul, and for the spirited part to obey and be its ally." (Republic IV.441e4-6.)
(Perseus Digital Library: Plato, Republic)