This is a good question. In Plato's view of the person, people have three parts. The mind or reasoning ability is highest part of a person. This is the place of philosophy. Then there is the spirit or the spirited part of the person. This can be viewed as the seat of emotions. Finally, there is the appetitive part of a person, which is motivated by desires.
Within this tripartite view of a person, Plato believes that base people are controlled by their appetitive parts. They simply do what they want to do without any thought of what is right or wrong. There is little reasoning ability. They even rush headlong into their desires, because the mind does not check their hearts or the spirited parts of a person.
Hence, according Plato, what is most important is that a person is controlled by the mind. A truly good man is ruled by reason or we can say philosophy. The mind, then, must control and reign in the appetitive part of a person.
Finally, as for the spirited part of a person, it is seen as neutral. It can be base, if it follows the appetitive part of a person, or it can be noble if it is guided by the mind.