At this point, a counterexample is raised by Adeimantus. This idealized picture of the philosopher is contradicted by reality: the course of study prescribed by Socrates is useless, and it produces cranks at best, debased persons at worst. Socrates agrees that this is what has actually happened, but he does not draw the conclusion that the well-trained philosopher is unfit to rule.
The reason philosophers become cranks is that their virtues are ignored by those who are themselves ignorant. In reality the leaders of the city rule by persuasion and force, and in such a context, the knowledge of the philosopher looks foolish. The reason philosophers are useless is that the rules of the city have no use for him.
Furthermore, the wise philosopher is rare, because those who are most talented are most easily corrupted, and they are capable of doing the greatest harm. So it is essential to give those of a superior nature the best teaching. This teaching is about the idea of the good. "It is the one and indispensible source of what is useful and excellent in justice and the other virtues." Nothing is worthwhile without possession of the good; it is the only thing it is profitable to possess.
But it is not easy to say what the good is, though everyone seeks it. "He intuits what the good is, but at the same time he is baffled, for the nature of the good is something he comprehends only inadequately." Those who govern should, however, know what the good is. Two possibilities are pleasure (sought by the multitude) and knowledge (sought by"those who claim greater refinement"). Socrates is implored to produce his own account of what the good is. An opinion about what the good is will not do, since "opinions divorced from knowledge are ugly [and] the best of opinions are blind."
The multiplicity can be seen but not thought, while the unity may be thought but not seen. The sun (governor of the world of things seen) is to vision and its visible objects as the good (governor of the intelligible order) is to reason and the objects of reason. "When the soul beholds the realm illuminated by the splendor of truth and reality, it knows and understands and so appears to possess reason. But when it turns its gaze to that region where darkness and light intermingle, to the transient world where all things are either quickening or dying, reason's edge is blunted. The soul becomes mired in opinion; and since opinion shifts from one direction to another, it appears that reason has vanished."
The role of the idea of the good, thus far, is two-fold. The objects of knowledge get their truth from the good, while the knower is able to know the truth through the good. "Because the idea of the good is the very cause of knowledge and truth, it is also the chief objective in the pursuit of knowledge." That is, it is not truth and knowledge which we seek in themselves (they are analogous to the object of sight and the seeing) but the cause of truth of knowledge (analogous to the sun). As the cause of the truth, the good is the object of the pursuit of knowledge. The greatest glory belongs to the good, which makes knowledge possible. The good also makes the objects of knowledge real, just as the sun nourishes the living things of the world. "It transcends being, exceeding all else in dignity and power."
On the side of the visible, the sub-division represents the difference between the mere images and physical objects, for example, between a shadow and a tree that casts it. On the side of the intelligible, the lower part consists of images. Our reasoning about them is not derived from and does not lead to fundamental truths. The higher part consists of forms, which are not images at all, but the models for the mental images.
For example, in geometry, images are subject to analysis, but the true object of the analysis is the originals (forms), "the square as such or the diagonal as such." The images used by the intellect, such as a cut-out of a disk, are physical objects which have their own images, such as shadows. The defect of this method is that it begins with hypotheses or assumptions, which are treated as given and self evident: "the several geometrical figures, the three kinds of angles" and other related things. But it does not tell us what these things are themselves. The process is one of understanding, but not reason: higher than opinion but lower than ultimate knowledge.
The highest level of intelligibility treats the hypothesis as mere means ("ladders and springboards") to ultimate knowledge which dispenses with them. They allow us to grasp the forms, and once the forms have been grasped, the intellect can concern itself with them alone. "It reasons only by using forms. It moves from forms through forms to forms. And it completes its journey in forms." This is the highest level of human cognition, the ultimate sort of knowledge.
An improvement would be made if the prisoners could view the fire-light casting the shadows. They would at least be confronted with the source (fire and puppets) of the images they see. But because they would be dazzled by the light, they would be unable to recognize the objects whose shadows were created by the fire. In fact, those objects would seem no more real than the shadows which were formerly thought to be the only real things. Further, the fire-light itself would be painful to behold, sending the prisoners fleeing back to their shadows.
So one does not go voluntarily toward the painful truth. A prisoner might be dragged painfully out into the sunlight, and again be dazzled and unable to see the realities outside the cave. Only habituation could allow him to become accustomed to the light. At first he would see shadows and reflections (cf. the line analogy), then physical objects, celestial bodies, and the sun itself. He would eventually recognize the dominant place of the sun in the order of things.
The liberated prisoner would feel pity for the others and attach no value to their ability to anticipate the course of shadows. If he were to go back, and his eyes were as yet unaccustomed to the dim light, he would be no match for the others in "shadow watching" and in turn be ridiculed by them.
Now the analogy is made explicit.
The cave represents our visible order
The fire represents our sun
The ascent represents "the soul's journey through the intelligible order"
The sun represents the cause of all things right and good
The idea of the good is the last to be seen, but it is the first in the order of being. To act wisely, one must know it. But once they do, they will be reluctant to engage in ordinary affairs, like the man who was returned to the cave.
Education does not come about, then, by simply transplanting knowledge into the learner. Rather, the learner must "turn away from the world of transient things toward the world of perpetual being, until finally one learns to endure the sight of its most radiant manifestation. This is what we call goodness, is it not?" Education is the art of directing the soul in this way, toward the eternal and away from the transient.
Wisdom can be used for good or ill. Those without knowledge of the good cannot use it to govern the city, while those who know the good do not wish to engage in the affairs of others, choosing to "remain above, refusing to go down again among those prisoners to share their labors and their rewards, whatever their worth may be." Although compelling them to govern seems to be wrong them, the leaders are educated for the purpose of governing, so they must govern. And they will govern well, because they have learned the shadowy nature of the ordinary world.
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