Socrates attempts to prove that the philosopher is best suited to rule. The philosopher as a lover of learning and truth is disinclined to attend to physical pleasures. Adeimantus interrupts to point out that most people think philosophers are vicious cranks, and the few good ones are useless to society. Socrates replies that this view is the result of faults in society, not in philosophers. Even a truly good man can not function well in a bad society.
Socrates criticizes the Sophists, the paid tutors whom he says teach conventional wisdom without considering whether it is true or not. The Sophists call what is pleasant "good," without really considering the truth of the matter.
The Sophists are not, according to Socrates, "real" philosophers since they are interested in earning their income by teaching what is easiest, rather than in truth.
The philosopher-king must be intelligent, reliable, and willing to lead a simple life. These qualities are rarely found in one person, and must be encouraged by education and the study of the Good. Just as visible objects must be illuminated in order to be seen, so objects of knowledge must be true. Just as light comes from the sun, so Truth comes from Goodness. Goodness as the source of truth makes it possible for the mind to know, just as light from the sun makes eyes able to see.
Socrates never actually defines Goodness. He creates an elaborate metaphor, using the sun as an analogy, and then builds on that analogy in the following metaphors of the Line, and the Cave.
Socrates introduces the metaphor of the Line. Think of a straight line divided into four sections or stages. The lowest stage on the line is Imagination, where images and reflections are thought real. The next stage is Belief, which deals with physical objects rather than reflections or images of them. The last two stages, Thought and Understanding, are both forms of knowledge. Thought uses the Forms, but it also relies on images, sense data, and hypotheses. Understanding relies only on the ideal Forms, beginning with the Form of the Good. The philosopher progresses through all four stages on the line, until he reaches Understanding. Only the philosopher reaches the last stage, where he understands the Form of the Good. Once he arrives at the form of the Good, all the other forms follow.
The line is a metaphor for the way the philosopher must ascend a series of levels from reality to knowledge, from immediate and passing sensation to the divine vision of the ideal forms. The philosopher moves from the impressions of objects perceived by the senses, to the ideal abstract forms of those objects.