Why does Glaucon mention the myth of the Ring of Gyges? What intuition of ours is he trying to jog?

In Book II, Glaucon tries to reinforce the challenge to justice that Socrates must meet in the remainder of the book. He argues that justice is the sort of good that is only desired for its consequences, not for its own sake. Justice, he claims, is a necessary evil that human beings endure out of fear and weakness. Because we can all suffer from one another’s injustices, he explains, we agree, as a society, to behave justly and thus avoid greater harm. Given the chance to escape reprisals, though, any human being would choose to be unjust rather than just.

In order to illustrate this point, Glaucon appeals to the Ring of Gyges. According to mythology, this ring has the special power to make its possessor invisible. Glaucon’s intention in invoking this magical entity is to argue that even the most just man only behaves as he does because of fear of reprisal. If such a man were able to behave unjustly with impunity—as he could if he were invisible—then he would do so.

Glaucon himself does not believe that justice is a necessary evil; he thinks that it is the highest form of good, the sort that is desired both for its own sake and for its consequences. His wish is that Socrates provide a compelling argument to this effect.

Why does Plato go to such lengths to prove that there are three distinct parts to the human soul? Explain both why he needs three aspects to the soul, and also why these aspects need to be distinct and independent from one another.

Plato applies the word ‘justice’ to both societies and individuals, and his overall strategy in The Republic is to first explicate the primary notion of political justice, and then to derive an analogous concept of individual justice. Plato defines political justice as being inherently structural. A society consists of three main classes of people—the producers, the auxiliaries, and the guardians; the just society consists in the right and fixed relationships between these three classes. Each of these groups must do the job appropriate to it, and only that job, and each must be in the right position of power and influence in relation to the others.

In Book IV, Plato demonstrates that these three classes of society have analogs in the soul of every individual. The soul is a tripartite entity. The just individual can be defined in analogy with the just society; the three parts of his soul are fixed in the requisite relationships of power and influence.

That is why Plato needs to show that there are three parts of the soul, but we can still ask why it is important for Plato to demonstrate that the three types of desire present in every individual correspond to three independent sources of desire.

This distinction of parts allows the three types of desire to be exerted simultaneously and to coexist with each other in both conflict and harmony. Political justice is a structural property, consisting in the realization of required relationships between three classes. The relationships constituting political harmony are fixed and static in the same sense as are the mathematical ratios constituting musical harmony. So in the just individual as well, though desires come and go, the relationship between the different sets of desires remains fixed and permanent.

Why does Plato banish the poets from his city?

After defining justice and proving its worth, Socrates turns his critical eye toward the poets. In a shocking move, he banishes nearly all poetry from his city (the only exceptions he makes are for hymns to gods and eulogies for famous men). Plato regrets this edict, feeling that it is an aesthetic sacrifice, but one necessitated by the greater good of the city; the poets, he feels, are too dangerous. He lays out three distinct, though related reasons for his harsh judgement.

His first gripe with the poets is that they deal in the least real things. Their wares are images, shadows, reflections. The objects of their art are, as Socrates puts it, far removed from “what is.” By “what is,” we understand the Forms—the unchanging, absolutes of the intelligible realm. The imperfect mutable copies of the Forms, sensible particulars such as trees, chairs, tables, flowers, are once removed from this most real realm. But the products of poetry are nothing but copies of these once-removed objects. Worse, since only the Forms can be objects of knowledge, the poets know nothing, though they are widely believed to have vast stores of knowledge.

In addition, poets make a practice of imitating the worst aspects of souls. They do not imitate the rational part, since this aspect is both hard to imitate and hard to understand. Instead, they imitate the appetitive part of the soul, and attempt primarily to gratify the appetites with laughter and cheap thrills.

Worst of all, poetry corrupts the soul, strengthening the appetitive part and weakening the rational. It encourages us to indulge in emotions like pity, amusement at base jokes, sympathy with sexual lusts. Because we feel these emotions vicariously through fictional characters, and not ourselves, we believe that we are safe. However, we do not realize that once we begin to allow these sorts of emotion reign they gain power and flourish. Soon we are feeling pity for ourselves, amusement at base events in our own life, and our own sexual lusts. Our appetitive part begins to gain control of the rational, and we are made unjust.