Why is it that the philosopher king is the best kind of ruler? How do these rulers arise?

Summary: Book VI, 502d-end

. . . in the intelligible realm it controls and provides truth and understanding, so that anyone who is to act sensibly in private or public must see it.

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Now Socrates turns to the final stage in the construction of the just city: the question of how to produce philosopher-kings.

He had mentioned in Book III that the guardians-in-training are subjected to many tests so that rulers are chosen from among them. One point of the test, he told us then, was to see who was most loyal to the city. Now we see that another major point of these tests is to determine who among them can tolerate the most important subject. The most important subject for a philosopher-king, it turns out, is the study of Form of the Good. It is in understanding the Form of the Good, in fact, that someone gains the highest level knowledge and thus becomes fit to be a philosopher king.

Socrates explains that the Form of the Good is not what is commonly held to be good. Some think that the highest good is pleasure, while the more sophisticated think that it is knowledge. In fact, it is neither of these, but Socrates cannot really say directly what it is. The best he can do is give an analogy—to say “what is the offspring of the good and most like it.” This analogy is the first in a string of three famous and densely interrelated metaphors that will stretch into the next book—the sun, the line, and the cave. In the course of developing these three metaphors, Socrates explains who the philosopher is, while working out his metaphysics and epistemology.

The sun, Socrates tells us, is to the visible realm what the Good is to the intelligible realm (the realm of Forms) in three respects. First, while the sun is the source of light, and hence, visibility in the visible realm, the Good is the source of intelligibility. Second, the sun is responsible for giving us sight, because it is only by incorporation of sun-like stuff into it that the eye is enabled to see. Similarly, the Good gives us the capacity for knowledge. Finally, the sun is responsible for causing things to exist (to “come to be”) in the visible realm. The sun regulates the seasons, it allows flowers to bloom, and it makes animals give birth. The Good, in turn, is responsible for the existence of Forms, for the “coming to be” in the intelligible realm. The Form of the Good, Socrates says, is “beyond being”—it is the cause of all existence.

The Form of the Good is responsible for all knowledge, truth, and for the knowing mind. It is the cause of the existence of the Forms in the intelligible realm, and the source for all that is good and beautiful in the visible realm. It is not surprising, then, that it is the ultimate aim of knowledge.

Yet not until we hear the next analogy do we understand just how important this Form of the Good is to knowledge. The analogy of the line is meant to illustrate the ways of accessing the world, the four grades of knowledge and opinion available to us. Imagine, says Socrates, a line broken into four segments. The bottom two segments represent our access to the visible realm, while the top two represent our access to the intelligible.

The lowest grade of cognitive activity is imagination. A person in the state of imagination considers images and reflections the most real things in the world. In Book X, we will see that art belongs to this class as well. It is less clear what Plato means by imagination, but he provides many interpretations.

The next stage on the line is belief. Belief also looks toward the realm of the visible, but it makes contact with real things. A person in the stage of belief thinks that sensible particulars are the most real things in the world.

Further up the line, there are two grades of knowledge: thought, and understanding. Although thought deals in Forms, it uses sensible particulars as images to aid in its reasoning, as when geometers use a picture of a triangle to help them reason about triangularity. Thought also relies on hypotheses, or unproven assumptions. Understanding uses neither of these crutches. Understanding is a purely abstract science. The reasoning involved deals exclusively with Forms, working with an unhypothetical first principle, which is the Form of the Good.

To reach understanding, an individual using the crutches necessary to thought, works his way up with philosophical dialectic toward the Form of the Good. Once you reach the Form of the Good, you have hit on your first principle, a universal proposition which makes all unproven hypotheses unnecessary. You now understand the Form of the Good, and all the other Forms as well. In a flash, you have reached the highest stage of knowledge.