Given that only philosophers can have knowledge, they are clearly the ones best able to grasp what is good for the city, and so are in the best position to know how to run and govern the city. If we only knew that they were virtuous—or at least no t inferior to others in virtue—then, Socrates’ friends agree, we could be sure that they are the ones most fit to rule. Luckily, we do know that philosophers are superior in virtue to everyone else. A philosopher loves truth more than anyt hing else (“philosopher” means “lover of truth or wisdom”); his entire soul strives after truth. This means that the rational part of his soul must rule, which means that his soul is just.

Adeimantus remains unconvinced. None of the philosophers he has ever known have been like Socrates is describing. Most philosophers are useless, and those that are not useless tend to be vicious. Socrates, surprisingly, agrees with Adeimantus’ condemnati on of the contemporary philosopher, but he argues that the current crop of philosophers have not been raised in the right way. Men born with the philosophical nature—courageous, high-minded, quick learne rs, with faculties of memory—are quickly preyed upon by family and friends, who hope to benefit from their natural gifts. They are encouraged to enter politics in order to win money and power by their parasitic family and friends. So they are inevitably led away from the philosophical life. In place of the natural philosophers who are diverted away from philosophy and corrupted, other people who lack the right philosophical nature, rush in to fill the gap and become philosophers when they have no right to be. These people are vicious.

The few who are good philosophers (those whose natures were somehow not corrupted, either because they were in exile, lived in a small city, were in bad health, or by some other circumstance) are considered useless because society has become antithetical to correct ideals. He compares the situation to a ship on which the ship owner is hard of hearing, has poor vision, and lacks sea-faring skills. All of the sailors on the ship quarrel over who should be captain, thou gh they know nothing about navigation. In lieu of any skill, they make use of brute force and clever tricks to get the ship owner to choose them as captain. Whoever is successful at persuading the ship owner to choose him is called a “navigator,” a “captain,” and “one who knows ships.” Anyone else is called “useless.” These sailors have no idea that there is a craft of navigation, or any knowledge to master in order to steer ships. In this scenario, Socrates points out, the true captain—the man who knows the craft of navigation—would be called a useless stargazer. The current situation in Athens is analogous: no one has any idea that there is real knowledge to be had, a craft to living. Instead, everyone tries to get ahead by clever, often unjust, tricks. Those few good philosophers who turn their sights toward the Forms and truly know things are deemed useless.

All that we need to make our city possible, Socrates concludes, is one such philosopher-king—one person with the right nature who is educated in the right way and comes to grasp the Forms. This, he believes, is not all that impossible.

Continuing with the defense of the philosopher, Plato asserts in this section that the philosopher is not only the sole possesor of knowledge, he is also the most virtuous of men. Plato indicates that the philosopher’s association with the Forms determines his virtue. By associating with what is ordered and divine (i.e., the Forms), the philosopher himself becomes ordered and divine in his soul. He patterns his soul after the Form of the Good.

Plato also offers a more intuitive explanation for why the philosopher is virtuous. Since all of him strives toward truth, his other desires are weakened. He has no real drive toward money, honor, pleasure, and so on. In short, he has none of the drives that can lead to immoral behavior. He would never be motivated to steal, lie, boast, act slavishly, or anything else of this sort. His emotions and appetites no longer provide a strong impetus toward vice.

This description makes it sound as if the philosopher’s soul is in a state of monopoly rather than a state of harmony. Instead of being ruled by reason, appetite and spirit are absent entirely. Elsewhere, however, Plato states th at the just man does retain all three sets of desires in robust forms. Though he loves truth most of all, he also desires pleasure and honor to a lesser extent. It is not clear how to reconcile this with the above picture. Perhaps we can simply ass ume that Plato was using excessively strong language when he spoke about the philosopher as if he had no drives other than the drive toward truth. But if the philosopher does retain his love of honor, money, and pleasure to a certain degree, then what guarantee do we have that he will never behave viciously? The likely answer is this: even if the philosopher might sometimes have desires that could lead to vicious acts, because reason dominates the other parts of his soul, he rar ely if ever acts on these desires.

Still, a question remains about the philosopher’s virtue. Allan Bloom points out that it sounds as if the philosopher is virtuous in a very strange way. He behaves virtuously mainly out of his preoccupation with ideas, and not out of the motivations we t ypically think of as marking the virtuous man. He is courageous, says Bloom, because he is constantly preoccupied with the eternal Forms and as a result is oblivious to life. He is not courageous because he is obedient to the city’s rules about what is fearful and what is not. He is moderate because he has an immoderate love of the truth, not because he restrains his desires. He is just in money matters because money plays only a small role in getting him what he wants, and so he cares little about it. We get no indication that he is just in money matters because he cares deeply about giving each person his due.

Taking off from this observation, Bloom divides virtue into two sorts—civic and intellectual—and argues that the philosophers only have the second kind. The civic virtues rise from the needs of the city; they are characteristics that aid in the goal of preserving the city and its inhabitants. The intellectual virtues stem from the needs of philosophy; they are characteristics that aid in gaining knowledge. Plato, he thinks, mistakenly (or perhaps deliberately) identifies these, so that he is able to claim that the philosopher is virtuous in the first sense, when he is only really virtuous in the second.

We might ask how important this mistake is, if it is really being made. If the philosopher is only virtuous in the second sense, but not the first, does that make him unfit to rule? It sounds as if it would. After all, if he lacks the civic virtues, th en he lacks the virtues that help him act for the good of the city. Plato is safe from this objection. What virtues does the ruler really need? So long as he behaves in the virtuous way, what do we ca re about his motivation? If he is acting only out of his love for wisdom, rather than out of his love for the city, does that harm the city in any way? What makes the philosopher the ideal ruler are not his virtues, but his knowledge. So long as his moral character does not pose a positive threat to the good of the city, we should not concern ourselves with the source of his virtues.

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.

Plato claims to have no way to explain the Form of the Good directly, but there is good reason to believe that he had something in mind as the highest good. Many scholars have believed that the Good was supposed to be identical with the One. The One represents unity, and unity, in turn, is closely related to determinacy. The advantage of this reading is that it helps to explain the connection between intelligibility and reality. Undoubtedly, something is only a real, determinate thing because it is a unified thing, a One. If this characteristic really is the Good, then it makes sense that the Good is responsible for all of reality. Nothing could be real, could exist, without this characteristic. In support of this reading, there are various points in the Republic where Plato emphasizes the importance of unity in the soul and in the city, remarking that a city without unity is not a real city.

A more likely candidate for the Form of the Good is harmony. Though Plato does praise unity at several points in the Republic, he praises harmony, order, and balance even more. Harmony between the three classes of society makes for a healthy, just city, and harmony in the soul makes for a healthy, just soul. When speaking of the superiority of the Forms, he often appeals to their supreme order, and explains that they make the philosopher virtuous by inspiring the same order in his soul. The good of each thing might simply be its appropriate harmony, order, balance, or proportion. What harmony could mean as applied to the Forms, since the Forms have no separate parts to harmonize, is less than obvious. But since Plato could not be more clear in his view that the Forms are the most ordered things, he must think there is some way to harmonize them. It might be this confusion—an inability to understand how the Forms can have harmony or order—that keeps Plato from being able to define the Form of the Good.

In the metaphor of the line, the most difficult stage to understand is imagination. Because in Book IX Plato indicates that art belongs to this category, many have understood imagination to refer to a state of mind in which products of art are viewed as the most real things. This state of mind is not as far-fetched as it might seem. Imagine a person who acquires his sense of self and of the world around him from images he sees on television or in movies. (In Plato’s time, the equivalent art forms would be epic poetry and tragic theater.) Such a person is not that difficult to imagine. You might even know someone like this.

Other scholars, though, have wondered whether Plato holds that art belongs to the category of imagination. There are other understandings of imagination that do not refer to art at all. On one such interpretation, imagination refers to a state in which our perceptions of the world are completely uncritical. In this state, we do not attempt to relate one perception to another. We see a reflection, and do not differentiate this from the object it is reflecting. Belief, then, would be the stage in which we correlate our perceptions, but fail to subject them to critical analysis. A related reading pegs imagination as a state in which we do not look for explanations, and belief as a state in which we do look for explanations but only in particular terms, rather than universal terms.

Thought and understanding are easier to pin down because Plato is more explicit about them. Thought is abstract reasoning that makes use of images and unproven assumptions. Geometry is the perfect example. In reasoning about triangularity, for example, geometers make use of diagrams of triangles. In order to prove theorems, they need to appeal to certain axioms that are taken as true without any attempt at proof. Understanding makes the axioms and hypotheses of thought unnecessary by seizing on a single universal proposition on which the entire body of knowledge can be based.