An important fact about Plato is that he was a Dualist. This means that he saw the world comprised of two sorts of things. One subject where this belief especially comes together, is his view on human beings.
Plato believed that a human was comprised of a body, which is physical, and a soul, which is spiritual. His ideas on the subject, although not originally his, became the first fully developed ideas in Western Philosophy of human beings consisting of two parts.
Like his teacher, Socrates, Plato believed humans to be essentially their souls. He believed that the body's desires were important, but not as much as the soul's.
Plato saw the soul as comprised of three parts and drew an analogy that compared it to a chariot.
Two horses represented the unruly parts of the soul. The first he called our spirit, or our emotions, easy to govern but if given free reign, will lead to an unhappy life.
Like a tug on the reign, education and ideals will easily convert emotions into moral virtues. The other horse he called our appetites. Controlling our appetites, such as bodily desires is not as easy. Desires clash with reason more so than emotion, and we often have difficulty overcoming their pull. Control, in forms of education, law and reason is important to lead a happy life.
The driver represents the rational element of the soul. It must be in control for the horses to work together.
Thus, to achieve a rich, full life, reason has to be in command of the irrational parts of the soul.
Not all people have these three elements to the same degree. Plato states that while there is one structure to human nature, the ratio of these elements produce different kinds of people and lives.
Source: "On Human Nature"
By Wall, T
P73 - 75