Socrates has now completed the main argument of the Republic; he has defined justice and shown it to be worthwhile. He turns back to the postponed question concerning poetry about human beings. In a surprising move, he banishes poets from the city. He has three reasons for regarding the poets as unwholesome and dangerous. First, they pretend to know all sorts of things, but they really know nothing at all. It is widely considered that they have knowledge of all that they write about, but, in fact, they do not. The things they deal with cannot be known: they are images, far removed from what is most real. By presenting scenes so far removed from the truth poets, pervert souls, turning them away from the most real toward the least.

Worse, the images the poets portray do not imitate the good part of the soul. The rational part of the soul is quiet, stable, and not easy to imitate or understand. Poets imitate the worst parts—the inclinations that make characters easily excitable and colorful. Poetry naturally appeals to the worst parts of souls and arouses, nourishes, and strengthens this base elements while diverting energy from the rational part.

Poetry corrupts even the best souls. It deceives us into sympathizing with those who grieve excessively, who lust inappropriately, who laugh at base things. It even goads us into feeling these base emotions vicariously. We think there is no shame in indulging these emotions because we are indulging them with respect to a fictional character and not with respect to our own lives. But the enjoyment we feel in indulging these emotions in other lives is transferred to our own life. Once these parts of ourselves have been nourished and strengthened in this way, they flourish in us when we are dealing with our own lives. Suddenly we have become the grotesque sorts of people we saw on stage or heard about in epic poetry.

Despite the clear dangers of poetry, Socrates regrets having to banish the poets. He feels the aesthetic sacrifice acutely, and says that he would be happy to allow them back into the city if anyone could present an argument in their defense.

Socrates then outlines a brief proof for the immortality of the soul. Basically, the proof is this: X can only be destroyed by what is bad for X. What is bad for the soul is injustice and other vices. But injustice and other vices obviously do not destroy the soul or tyrants and other such people would not be able to survive for long. So nothing can destroy the soul, and the soul is immortal.

Once Socrates has presented this proof, he is able to lay out his final argument in favor of justice. This argument, based on the myth of Er, appeals to the rewards which the just will receive in the afterlife. According to the myth, a warrior named Er is killed in battle, but does not really die. He is sent to heaven, and made to watch all that happens there so that he can return to earth and report what he saw. He observes an eschatalogical system which rewards virtue, particularly wisdom. For 1000 years, people are either rewarded in heaven or punished in hell for the sins or good deeds of their life. They are then brought together in a common area and made to choose their next life, either animal or human. The life that they choose will determine whether they are rewarded or punished in the next cycle. Only those who were philosophical while alive, including Odysseus who chooses to be reborn as a swan, catch on to the trick of how to choose just lives. Everyone else hurtles between happiness and misery with every cycle.

In Book X, Plato at last pits philosophy-based education in confrontation with traditional poetry-based education. Plato has justified philosophy and the philosopher and now he displays them in relation to their rivals—the people who are currently thought most wise and knowledgeable—the poets.

The myth, in appealing to reward and punishment, represents an argument based on motivations Plato earlier dismissed. Glaucon and Adeimantus had specifically asked him to praise justice without appealing to these factors. Why is he now doing exactly that?

Allen Bloom suggests that the inclusion of this myth is connected to the distinction between philosophical virtue and civic virtue. Philosophical virtue is the kind of virtue the philosopher possesses, and this kind of virtue differs from the virtue of the normal citizen. So far, says Bloom, Plato has only shown that philosophical virtue is worthy in itself. He has not shown that civic virtue is worthy. Since Glaucon and Adeimantus and countless others are not capable of philosophical virtue, he must provide them with some reason to pursue their own sort of virtue. With the contrast between philosophical and civic virtue in mind, Plato describes the thousand year cycles of reward and punishment that follow just and unjust lives.

Yet on our understanding of what makes any virtue worthwhile—its connection to the Forms—Plato has sufficiently demonstrated the worth of both sorts of virtue. Philosophical virtue might be more worthwhile because it not only imitates the Forms, but aims at and consorts with them, but civic virtue is worthwhile as well because it involves bringing the Forms into your life by instituting order and harmony in your soul. Bloom, though, also has another plausible hypothesis for why Plato included the myth of Er, and this one coheres well with our understanding of justice’s worth. The myth of Er, Bloom explains, illustrates once again the necessity of philosophy. The civic virtues alone are not enough. Only the philosophers know how to choose the right new life, because only they understand the soul and understand what makes for a good life and a bad one. The others, who lack this understanding, sometimes choose right and sometimes wrong. They fluctuate back and forth between good lives and miserable ones. Since every soul is responsible for choosing his own life, every person must take full responsibility for being just or unjust. We willingly choose to be unjust because of our ignorance of what makes for a just or unjust soul. Ignorance, then, is the only true sin, and philosophy the only cure.