As an account of political organization on the larger scale, Plato's defense of an aristocratic government was unlikely to win broad approval in democratic Athens. He used the characters Glaucon and Adeimantus to voice practical objections against the plan. They are especially concerned (as Plato's Athenian contemporaries may well have been) with some of its provisions for the guardian class, including the participation of both men and women, the elimination of families, and the education of children.
Most fifth century Greeks, like many 20th century Americans, supposed that natural differences between males and females of the human species entail a significant differentiation of their proper social roles. Although Plato granted that men and women are different in height, strength, and similar qualities, he noted that these differences are not universal; that is, for example, although it may be true that most men are taller than most women, there are certainly some women who are taller than many men. What is more, he denied that there is any systematic difference between men and women with respect to the abilities relevant to guardianship – the capacity to understand reality and make reasonable judgments about it. (Republic 454d) Thus, Plato maintained that prospective guardians, both male and female, should receive the same education and be assigned to the same vital functions within the society.
In addition, Plato believed that the interests of the state are best preserved if children are raised and educated by the society as a whole, rather than by their biological parents. So he proposed a simple (if startlingly unfamiliar) scheme for the breeding, nurturing, and training of children in the guardian class. (Note that the same children who are not permitted to watch and listen to "dangerous" art are encouraged to witness first-hand the violence of war.) The presumed pleasures of family life, Plato held, are among the benefits that the higher classes of a society must be prepared to forego.
A general objection to the impracticability of the entire enterprise remains. Even if we are persuaded that Plato's aristocracy is the ideal way to structure a city-state, is there any possibility that it will actually be implemented in a human society? Of course there is a sense in which it doesn't matter; what ought to be is more significant for Plato than what is, and philosophers generally are concerned with a truth that transcends the facts of everyday life.
But Plato also believed that an ideal state, embodying the highest and best capabilities of human social life, can really be achieved, if the right people are put in charge. Since the key to the success of the whole is the wisdom of the rulers who make decisions for the entire city, Plato held that the perfect society will occur only when kings become philosophers or philosophers are made kings. (Republic 473d)
Only those with a philosophical temperament, Plato supposed, are competent to judge between what merely seems to be the case and what really is, between the misleading, transient appearances of sensible objects and the the permanent reality of unchanging, abstract forms. Thus, the theory of forms is central to Plato's philosophy once again: the philosophers who think about such things are not idle dreamers, but the true realists in a society. It is precisely their detachment from the realm of sensory images that renders them capable of making accurate judgments about the most important issues of human life.
Thus, despite prevalent public skepticism about philosophers, it is to them that an ideal society must turn for the wisdom to conduct its affairs properly. But philosophers are made, not born. So we need to examine the program of education by means of which Plato supposed that the future philosopher-kings can acquire the knowledge necessary for their function as decision-makers for the society as a whole.
Since an ideal society will be ruled by those of its citizens who are most aware of what really matters, it is vital to consider how that society can best raise and educate its philosophers. Plato supposed that under the usual haphazard methods of childrearing, accidents of birth often restrict the opportunities for personal development, faulty upbringing prevents most people from achieving everything of which they are capable, and the promise of easy fame or wealth distracts some of the most able young people from the rigors of intellectual pursuits. But he believed that those with the greatest ability – that is, people with a natural disposition fit for philosophical study – must receive the best education, engaging in a regimen of mental discipline that grows more strict with every passing year of their lives.
The highest goal in all of education, Plato believed, is knowledge of the Good; that is, not merely an awareness of particular benefits and pleasures, but acquaintance with the Form itself. Just as the sun provides illumination by means of which we are able to perceive everything in the visual world, he argued, so the Form of the Good provides the ultimate standard by means of which we can apprehend the reality of everything that has value. (Republic 508e) Objects are worthwhile to the extent that they participate in this crucial form.
So, too, our apprehension of reality occurs in different degrees, depending upon the nature of the objects with which it is concerned in each case. Thus, there is a fundamental difference between the mere opinion (Gk. δοξα [dóxa]) we can have regarding the visible realm of sensible objects and the genuine knowledge (Gk. επιστημη [epistêmê]) we can have of the invisible realm of the Forms themselves. In fact, Plato held that each of these has two distinct varieties, so that we can picture the entire array of human cognition as a line divided proportionately into four segments. (Republic 509d)
At the lowest level of reality are shadows, pictures, and other images, with respect to which imagination (Gk. εικασια [eikásia]) or conjecture is the appropriate degree of awareness, although it provides only the most primitive and unreliable opinions.
The visible realm also contains ordinary physical objects, and our perception of them provides the basis for belief (Gk. πιστις [pístis]), the most accurate possible conception of the nature and relationship of temporal things.
Moving upward into the intelligible realm, we first become acquainted with the relatively simple Forms of numbers, shapes, and other mathematical entities; we can achieve systematic knowledge of these objects through a disciplined application of the understanding (Gk. διανοια [diánoia]).
Finally, at the highest level of all, are the more significant Forms – true Equality, Beauty, Truth, and of course the Good itself. These permanent objects of knowledge are directly apprehended by intuition (Gk. νοησις [nóêsis]), the fundamental capacity of human reason to comprehend the true nature of reality.
Plato recognized that the picture of the Divided Line may be difficult for many of us to understand. Although it accurately represents the different levels of reality and corresponding degrees of knowledge, there is a sense in which one cannot appreciate its full significance without first having achieved the highest level. So, for the benefit of those of us who are still learning but would like to grasp what he is talking about, Plato offered a simpler story in which each of the same structural components appears in a way that we can all comprehend at our own level. This is the Allegory of the Cave.
Suppose that there is a group of human beings who have lived their entire lives trapped in a subterranean chamber lit by a large fire behind them. Chained in place, these cave-dwellers can see nothing but shadows (of their own bodies and of other things) projected on a flat wall in front of them. Some of these people will be content to do no more than notice the play of light and shadow, while the more clever among them will become highly skilled observers of the patterns that most regularly occur. In both cases, however, they cannot truly comprehend what they see, since they are prevented from grasping its true source and nature. (Republic 514a)
Now suppose that one of these human beings manages to break the chains, climb through the torturous passage to the surface, and escape the cave. With eyes accustomed only to the dim light of the former habitation, this individual will at first be blinded by the brightness of the surface world, able to look only upon the shadows and reflections of the real world. But after some time and effort, the former cave-dweller will become able to appreciate the full variety of the newly-discovered world, looking at trees, mountains, and (eventually) the sun itself.
Finally, suppose that this escapee returns to the cave, trying to persuade its inhabitants that there is another, better, more real world than the one in which they have so long been content to dwell. They are unlikely to be impressed by the pleas of this extraordinary individual, Plato noted, especially since their former companion, having travelled to the bright surface world, is now inept and clumsy in the dim realm of the cave. Nevertheless, it would have been in the best interest of these residents of the cave to entrust their lives to the one enlightened member of their company, whose acquaintance with other things is a unique qualification for genuine knowledge.
Plato seriously intended this allegory as a representation of the state of ordinary human existence. We, like the people raised in a cave, are trapped in a world of impermanence and partiality, the realm of sensible objects. Entranced by the particular and immediate experiences these things provide, we are unlikely to appreciate the declarations of philosophers, the few among us who, like the escapee, have made the effort to achieve eternal knowledge of the permanent forms. But, like them, it would serve us best if we were to follow this guidance, discipline our own minds, and seek an accurate understanding of the highest objects of human contemplation.
Having already described the elementary education and physical training that properly occupy the first twenty years of the life of prospective guardians, Plato applied his account of the structure of human knowledge in order to prescribe the disciplined pursuit of their higher education.
It naturally begins with mathematics, the vital first step in learning to turn away from the realm of sensible particulars to the transcendent forms of reality. Arithmetic provides for the preliminary development of abstract concepts, but Plato held that geometry is especially valuable for its careful attention to the eternal forms. Study of the (mathematical, not observational) disciplines of astronomy and harmonics encourage the further development of the skills of abstract thinking and proportional reasoning.
Only after completing this thorough mathematical foundation are the future rulers of the city prepared to begin their study of philosophy, systematizing their grasp of mathematical truth, learning to recognize and eliminate all of their presuppositions, and grounding all genuine knowledge firmly on the foundation of their intuitive grasp of the reality of the Forms. Finally, an extended period of apprenticeship will help them to learn how to apply everything they have learned to the decisions necessary for the welfare of the city as a whole. Only in their fifties will the best philosophers among them be fit to rule over their fellow-citizens.
In order to explain the distinction between justice and injustice more fully, Plato devoted much of the remainder of The Republic to a detailed discussion of five different kinds of government (and, by analogy, five different kinds of person), ranked in order from best to worst:
A society organized in the ideally efficient way Plato has already described is said to have an aristocratic government. Similarly, an aristocratic person is one whose rational, spirited, and appetitive souls work together properly. Such governments and people are the most genuine examples of true justice at the social and personal levels.
In a defective timocratic society, on the other hand, the courageous soldiers have usurped for themselves the privilege of making decisions that properly belongs only to its better-educated rulers. A timocratic person is therefore someone who is more concerned with belligerently defending personal honor than with wisely choosing what is truly best.
In an oligarchic government, both classes of guardian have been pressed into the service of a ruling group comprising a few powerful and wealthy citizens. By analogy, an oligarchic personality is someone whose every thought and action is devoted to the self-indulgent goal of amassing greater wealth.
Even more disastrously, a democratic government holds out the promise of equality for all of its citizens but delivers only the anarchy of an unruly mob, each of whose members is interested only in the pursuit of private interests. The parallel case of a democratic person is someone who is utterly controlled by desires, acknowledging no bounds of taste or virtue in the perpetual effort to achieve the momentary satisfaction that pleasure provides.
Finally, the tyrranic society is one in which a single individual has gained control over the mob, restoring order io place of anarchy, but serving only personal welfare instead of the interests of the whole city. A tyrranic person, then, must be one whose entire life is focussed upon the satisfaction of a single desire at the expense of everything else that truly matters. Governments and people of this last variety are most perfectly unjust, even though they may appear to be well-organized and effective.
Although Plato presents these five types of government or person as if there is a natural progression from each to the next, his chief concern is to exhibit the relative degree of justice achieved by each. The most perfect contrast between justice and injustice arises in a comparison between the aristocratic and the tyrranic instances.
Thus, we are finally prepared to understand the full force of Plato's answer to the original challenge of showing that justice is superior to injustice. He offered three arguments, each of which is designed to demonstrate the intrinsic merits of being a just person.
First, Plato noted that the just life of an aristocratic person arises from an effortless harmony among internal elements of the soul, while the unjust life of a tyrranic person can maintain its characteristic imbalance only by the exertion of an enormous effort. Thus, it is simply easier to be just than to be unjust. (Republic 580a) This argument makes sense even independently of Plato's larger theory; it is a generalized version of the fairly common notion that it is easier to be honest than to keep track of the truth along with a number of false stories about it.
Second, Plato claimed that tyrranic individuals can appreciate only pleasures of the body, monetary profits, and the benefits of favorable public reputation, all of which are by their nature transitory. Aristocratic people, on the other hand, can accept these things in moderation but also transcend them in order to enjoy the delights of intellectual achievement through direct acquaintance with the immutable Forms. (Republic 583a) This argument relies more heavily upon adoption of Plato's entire theory of human nature, as developed in The Republic and other dialogues; it is likely to influence only those who have already experienced the full range of intellectual advantages for themselves.
Finally, Plato resorted to myth (just as he had at the close of the Phaedo by imagining that justice will be rewarded with steady progression in a series of lives hereafter. This "Myth of Er" isn't philosophical argument at all. Even if it were literally true and demonstrable that the just are rewarded in the afterlife, that would be only an extrinsic motive for being just, not a proof of its intrinsic value.
Although it is a masterly treatment of human nature and politics, The Republic was not Plato's only discussion of these significant issues. His dialogue Gorgias includes an eloquent appeal on behalf of the life of justice and personal non-violence in all things. The Statesman devotes extended attention to the practical matter of securing effective government under the less-than-ideal conditions most of us commonly face. And the unfinished Λεγεισ (Laws) is a lengthy analysis of the history of Athenian political life.