Earlier, Plato had divided his city into three parts: the guardians, the auxiliaries, and the craftsmen. Now, he divides a man into three similar parts, namely; reason, desires, and emotions. The reasoning behind this division seems to be that it can sometimes appear that a man is behaving in two opposite ways at once but, in fact, it is a case of his different parts being in conflict with one another.
It seems that Freud took a page from Plato when he divided the human psyche into the id, ego, and superego. However, Plato seems to be more interested in how a man’s mind must be in order for him to be just, rather than the psychology of how a mind is composed.
Plato, again, likens the three components of a man’s mind to the three divisions of his city. He claims that the city is just when each of the three classes performs its proper function. Likewise, he says, the same is true of a man. If the three parts of a man perform their proper role and do not interfere with one another, the mind will be in good order, and a just man is one whose mind is in good order. Just as he attempts to put everything into its proper place in his city, Plato attempts to put everything into its proper place in a man.
He goes on to argue that reason, like the guardians in the city, must always remain in control. In a just man, reason will always win out when there is a struggle or conflict between any of the three parts.