Plato wants to educate the guardians of his ideal republic so that they are both high-spirited and gentle. He proposes to do this by training both their souls and bodies. His curriculum consists of music, which includes literature, and gymnastics (physical training and sports). It is possible to blend these traits because I know many people who are good at music and sports. People can have two different qualities within themselves even if the qualities are completely different.
The elders are to rule the younger. The leaders are selected after being tested throughout their lives. They are put through many tests in order to see if they will make good leaders or not. If they prove themselves by remaining loyal to the State, always fulfilling their duties, and overcoming challenges, then they are chosen as leaders. The "myth of metals" reinforces this because it says that God made some people gold, some silver, and some brass. The gold are the guardians, the silver are the auxiliaries, and the brass are the husbandmen and artisans. The metal that makes up a person does not depend on what kind of metal a person's parents were. This means that each individual has his own talents and must prove himself if he wishes to become a leader.
