Therefore, the soul has to be unified to perform as a one coherent system, and such unifier for it is the reason. But was it only the spirited to decide on the way to follow, and the reason to regulate and guide the soul, it would just have "a list of things" to gain and a method to get them, but be out of power to reach out for and achieve them. However, there is the Desire, which stimulates all parts of the soul and encourages it to further development and finally leads to soul's satisfaction. Now, with these three elements equalized (and ruled by the reason), the soul exemplifies a harmonious unity, full of justice, and ready for a quest for the good.
In his theory, the philosopher compares the soul and its harmony first with a human body and second with the state. Collating the soul with a body, he says that the body will not function properly unless all its systems and organs work together to sustain the healthy state of the body. When Plato says the soul and the state are quite similar to each other, he implies that all parts of these structures have to do their own jobs and thus to provide all the necessities for the existence in harmony. For example, wise philosophers should govern the state as the rational beginning rules the soul; auxiliaries do the things, required for the state (as cases in point, we can consider warriors, who represent the power and defend the state, and peasants, who provide it with food) - the same what the spirited beginning does for the soul.
Well, Plato wouldn't have been as splendid a philosopher as he was, if he hadn't given reasons for treating the rational beginning as the balancing instrument of the soul. He did. The explanation actually comes from his conviction that the good (or the Form of the good) can be perceived only by knowledge. That is so due to the assumption Plato was sure in: the good is not a set of laws created by each person and consequently always different, but a constant form, the same for every one, which can be obtained only by means of the rational, by knowledge.
