Having identified the just city and the just soul, Socrates now wants to identify four other constitutions of city and soul, all of which are vicious to varying degrees. But before he can get anywhere in this project, Polemarchus and Adeimantus interrupt him. They would like him to return to the statement he made in passing about sharing spouses and children in common. Socrates launches into a lengthy discussion about the lifestyle of the guardians.

In the first of several radical claims that he makes in this section Socrates declares that females will be reared and trained alongside males, receiving the same education and taking on the same political roles. Though he acknowledges that in many respects men and women have different natures, he believes that in the relevant respect—the division among appetitive, spirited, and rational people—women fall along the same natural lines as men. Some are naturally appetitive, some naturally spirited, and some naturally rational. The ideal city will treat and make use of them as such.

Socrates then discusses the requirement that all spouses and children be held in common. For guardians, sexual intercourse will only take place during certain fixed times of year, designated as festivals. Males and females will be made husband and wife at these festivals for roughly the duration of sexual intercourse. The pairings will be determined by lot. Some of these people, those who are most admirable and thus whom we most wish to reproduce, might have up to four or five spouses in a single one of these festivals. All the children produced by these mating festivals will be taken from their parents and reared together, so that no one knows which children descend from which adults. At no other time in the year is sex permitted. If guardians have sex at an undesignated time and a child results, the understanding is that this child must be killed.

To avoid rampant unintentional incest, guardians must consider every child born between seven and ten months after their copulation as their own. These children, in turn, must consider that same group of adults as their parents, and each other as brothers and sisters. Sexual relations between these groups is forbidden.

Socrates explains that these rules of procreation are the only way to ensure a unified city. In most cities the citizens’ loyalty is divided. They care about the good of the whole, but they care even more about their own family. In the just city, everyone is considered as family and treated as such. There are no divided loyalties. As Socrates puts it, everyone in the city says “mine” about the same things. The city is unified because it shares all its aims and concerns.

The final question to be asked is whether this is a plausible requirement—whether anyone can be asked to adhere to this lifestyle, with no family ties, no wealth, and no romantic interludes. But before answering this question, Socrates deals with a few other issues pertaining to the guardians’ lifestyle, all of them relating to war. He states that children training to become guardians should be taken to war so they can watch and learn the art as any young apprentice does. He recommends that they be put on horseback so that they can escape in the case of defeat. He also explains that anyone who behaves cowardly in war will be stripped of their role as a guardian. He ends by discussing the appropriate manner in which to deal with defeated enemies. When it comes to Greek enemies, he orders that the vanquished not be enslaved and that their lands not be destroyed in any permanent way. This is because all Greeks are really brothers, and eventually there will be peace between them again. When it comes to barbarian—i.e., non-Greek—enemies, anything goes.

Plato advocates the equal education of women in Book V, but it would be inaccurate to think that Plato believed in the modern notion of equality between the sexes. He states in this section that women are inferior to men in all ways, including intellect. He could not have thought that all women were inferior to all men, or else dividing women into the three classes would make no sense. Instead, he believed that within each class the women are inferior to the men. So, for instance, guardian women would be superior to men of the two other classes, but inferior to most men of their own class.

With regard to the larger topic of family life, we might ask why common families are limited to the guardian class. Given that this arrangement is offered as a guarantee for patriotism, a preemptive strike against divided loyalties, why should it only apply to this class of society? The first thing to point out in relation to this topic is that the restrictions on family life are probably meant to apply to both the guardian and the auxiliary classes. These two classes are, after all, raised and educated together until adolescence when the rulers are chosen out as the best among the group, so chances are that their lifestyles are the same as well. Plato is often sloppy with the term “guardian,” using it to apply sometimes only to the rulers and other times to both rulers and warriors. It is likely that the restriction on personal wealth also applies to auxiliaries.

The only class left out of this requirement is the producers. Since the producers have little to do with the political life of the city—they do not have to make any decisions pertaining to the city, or to fight on behalf of the city—their patriotism does not matter. Just as we saw that a courageous farmer does no good for the city as a whole, a patriotic craftsman or doctor is irrelevant from the standpoint of the society’s good. The producers’ only political task is to obey.

Socrates has procrastinated long enough and must explain how guardians could be compelled to live in this bizarre way. His response is the most radical claim yet. Our system is only possible, he says, if the rulers are philosophers. Thu s he introduces the concept of the philosopher-king, which dominates the rest of the Republic.

To back up this shocking claim, Socrates must explain, of course, what he means by the term “philosopher.” Clearly he cannot mean to refer to the sort of people who are currently called “philosophers,” since these people do not seem fit to rule. The first step in introducing the true philosopher is to distinguish these special people from a brand of psuedo-intellectuals whom Socrates refers to as the “lovers of sights and sounds.” The lovers of sights and sounds are aesthetes, diletta ntes, people who claim expertise in the particular subject of beauty.

In the distinction of the philosopher from the lover of sights and sounds the theory of Forms first enters the Republic. Plato does not explain through Socrates what the Forms are but assumes that his audience is familiar with the theory. Forms, we learn in other Platonic dialogues, are eternal, unchanging, universal absolute ideas, such as the Good, the Beautiful, and the Equal. Though Forms cannot be seen—but only grasped with the mind—they are responsible for making the things we sense around us into the sorts of things the y are. Anything red we see, for instance, is only red because it participates in the Form of the Red; anything square is only square because it participates in the Form of the Square; anything beautiful is only beautiful because it participates in the Form of Beauty, and so on.

What makes philosophers different from lovers of sights and sounds is that they apprehend these Forms. The lovers of sights and sounds claim to know all about beautiful things but cannot claim to have any knowledge of the Form of the Beautiful—nor do they even recognize that there is such a thing. Because the lovers of sights and sounds do not deal with Forms, Socrates claims, but only with sensible particulars—that is, the particular things we sense around us—they ca n have opinions but never knowledge. Only philosophers can have knowledge, the objects of which are the Forms.

In order to back up this second radical claim—that only philosophers can have knowledge—Socrates paints a fascinating metaphysical and epistemological picture. He divides all of existence up into three classes: what is completely, what is in no way, and what both is and is not. What is completely, he tells us, is completely knowable; what is in no way is the object of ignorance; what both is and is not is the object of opinion or belief. The only things that are completely are the Forms. Only the Form of the B eautiful is completely beautiful, only the Form of Sweetness is completely sweet, and so on. Sensible particulars both are and are not. Even the sweetest apple is also mixed in with some sourness—or not-sweetness. Even the most beautiful woman is plain—or not-beautiful—when judged against certain standards. So we can only know about Forms, and not about sensible particulars. That is why only philosophers can have knowledge, because only they have access to the Forms.

In this section Plato makes one of the most important claims of the book: only the philosopher has knowledge. In fact, if we read the Republic as a defense of the activity of philosophy, as Allan Bloom suggests, then this might be viewed as the most important claim. It explains why philosophy is crucial to the life of the city, rather than a threat to society.

The argument for this claim proceeds, roughly, as follows. Only “what is completely” is completely knowable. Only the Forms count as “what is completely.” Only philosophers have access to the Forms. Only the philosophers have kno wledge.

That only the Forms qualify as “what is completely” is a radical and contentious idea. Can a beautiful woman be completely beautiful? Is it not the case that she is only beautiful according to some standards, and not according to others? Compared to a goddess, for instance, she would probably appear plain. So the beautiful woman is not completely beautiful. No sensible particular can be completely anything—judged by some standards, or viewed in some way, it will lack that quality. It will certainly lose the quality over time. Nothing is sweet forever; fruit eventually withers, rots, dessicates. Nothing is beautiful forever; objects eventually corrode, age, or perish. The Form of Beauty is nothing but pure beauty that lasts without alteration forever . In Plato’s conception, all Forms possess their singular qualities completely, eternally, and without change.

That only “what is completely” is completely knowable is a difficult idea to accept, even when we understand what Plato means to indicate by speaking of the Forms. Consider our beautiful woman. Remember that she is at the same time both beautiful and not beautiful and that her beauty must inevitably fade. So how can we know that she is beautiful, when she is not completely or permanently beautiful? To think that she is beautiful cannot amount to knowledge if it is partially false. But why can we not say that we know exactly in what way she is beautiful and in what ways not, that we know the whole picture? The reason that th is does not work is that our beautiful woman is a changing entity, as are all sensible particulars. Since she herself is a changing entity, our grasp of her, if it is correct, has to change as well. Plato is adamant that knowledge does not chan ge. Knowledge for Plato, as for Aristotle and many thinkers since, consists in eternal, unchanging, absolute truths, the kind that he would count as scientific. Since knowledge is limited to eternal, unchanging, absolute truths, it cannot app ly to the ever changing details of the sensible world. It can only apply to what is completely—to what is stable and eternally unchanging.

Plato, some might claim, is making a mistake in leaping from the claim that knowledge must apply to stable, unchanging truths to the claim that knowledge only applies to Forms. His student Aristotle also believed that knowledge is limited to eternal and absolute truths, but he found a way to let knowledge apply to the world we observe around us by limiting knowledge to classes or kinds. We can have knowledge, in Aristotle’s view, about human beings, but not about any particular human being. Classes, he realized, are stable and eternal, even if the particular entities that make them up are not.

In this section there are distinct echoes of earlier philosophers. In dividing all of existence up into three classes (what is completely, what is not at all, and what both is and is not), Plato draws on elements of pre-Socratic theories and synthesizes these elements into a coherent worldview. Parmenides is echoed in the extremes: in what is completely and in what is not at all. Parmenides spoke a great deal ab out “what is” and “what is not.” He argued that all that exists—“what is”—is a single, unchanging, eternal thing—an entity that in many ways resembles the Forms (though it differs from the Forms, for instance, in that Parmenides’ “what is” was a singular entity, while Plato allows for multiple Forms). Everything else, he said, is not at all. While Parmenides would have sympathized with Plato’s two extremes, he would have strenuously objected to the existence of the middle realm—what both is and is not. By partaking of both “what is” and “what is not,” this realm would have severely violated logic.

This realm, though, does have strong ties to another pre-Socratic philosopher, Heraclitus. One of Heraclitus’ main doctrines was a theory concerning unity of opposites: the idea that whatever is beautiful is also ugly, whatever up also down, and so forth.

He believed that the entire world was composed out of these unities of opposites and that the key to understanding nature was to understand how these opposites cohered.

It is not surprising to find Plato drawing on these two thinkers, since he studied with students of both Parmenides and Heraclitus before he founded his Academy.