Republic, Book II
First Question: Thrasymachus' Failure
Book I of Republic sets the stage at the start of Book II for Glaucon and Adeimantus' central challenge to Socrates which launches the discussion more thoroughly than Thrasymachus' inept challenge ever could have. Glaucon goes first,
...I think that Thrasymachus gave up before he had to, charmed by you as if he were a snake. But I’m not yet satisfied by the argument on either side. I want to know what justice and injustice are and what power each itself has when it’s by itself in the soul. I want to leave out of account their rewards and what comes from each of them. So, if you agree, I’ll renew the argument of Thrasymachus. First, I’ll state what kind of thing people consider justice to be and what its origins are. Second, [c] I’ll argue that all who practice it do so unwillingly, as something necessary, not as something good. Third, I’ll argue that they have good reason to act as they do, for the life of an unjust person is, they say, much better than that of a just one.
It isn’t, Socrates, that I believe any of that myself. I’m perplexed, indeed, and my ears are deafened listening to Thrasymachus and countless others. But I’ve yet to hear anyone defend justice in the way I want, proving that it is better than injustice. I want to hear it praised by itself, and I think that [d] I’m most likely to hear this from you. Therefore, I’m going to speak at length in praise of the unjust life, and in doing so I’ll show you the way I want to hear you praising justice and denouncing injustice. But see whether you want me to do that or not.
I want that most of all. Indeed, what subject could someone with any understanding enjoy discussing more often?
As we discover, Thrasymachus' collapse in arguing with Socrates is but a prelude to a much more penetrating analysis by Glaucon, precisely the one he outlines above. Why, though, would Plato spend valuable text on Thrasymachus? Why not simply start with Glaucon's challenge in Book II? What, in other words, is the point of Book I?
Second Question: What is Justice Taken to Be?
Glaucon proceeds thusly,
Excellent. Then let’s discuss the first subject I mentioned—what justice [e] is and what its origins are.
They say that to do injustice is naturally good and to suffer injustice bad, but that the badness of suffering it so far exceeds the goodness of doing it that those who have done and suffered injustice and tasted both, but who lack the power to do it and avoid suffering it, decide that it is  profitable to come to an agreement with each other neither to do injustice nor to suffer it. As a result, they begin to make laws and covenants, and what the law commands they call lawful and just. This, they say, is the origin and essence of justice. It is intermediate between the best and the worst. The best is to do injustice without paying the penalty; the worst is to suffer it without being able to take revenge. Justice is a mean between these two extremes. People value it not as a good but because they are [b] too weak to do injustice with impunity. Someone who has the power to do this, however, and is a true man wouldn’t make an agreement with anyone not to do injustice in order not to suffer it. For him that would be madness. This is the nature of justice, according to the argument, Socrates, and these are its natural origins. We can see most clearly that those who practice justice do it unwillingly [c] and because they lack the power to do injustice, if in our thoughts we grant to a just and an unjust person the freedom to do whatever they like. We can then follow both of them and see where their desires would lead. And we’ll catch the just person red-handed travelling the same road as the unjust. The reason for this is the desire to outdo others and get more and more. This is what anyone’s nature naturally pursues as good, but nature is forced by law into the perversion of treating fairness with respect.
Without invoking Game Theory and the Prisoner's Dilemma, Glaucon here invites us to consider a somewhat contractarian view of justice. How might his account be bolstered by the Prisoner's Dilemma? What does it say about the concept of justice if it really is better to manage (by power, as Thrasymachus would have it, or by means of manipulation in general) to be a free-rider/defector from the social contract?
Third Question: The Ring of Gyges
In a famous passage, Glaucon constructs a thought experiment further refined by his brother Adeimantus:
The freedom I mentioned would be most easily realized if both people had the power they say the ancestor of Gyges of Lydia possessed. The [d] story goes that he was a shepherd in the service of the ruler of Lydia. There was a violent thunderstorm, and an earthquake broke open the ground and created a chasm at the place where he was tending his sheep. Seeing this, he was filled with amazement and went down into it. And there, in addition to many other wonders of which we’re told, he saw a hollow bronze horse. There were window-like openings in it, and, peeping in, he saw a corpse, which seemed to be of more than human size, wearing [e] nothing but a gold ring on its finger. He took the ring and came out of the chasm. He wore the ring at the usual monthly meeting that reported to the king on the state of the flocks. And as he was sitting among the others, he happened to turn the setting of the ring towards himself to the inside of his hand. When he did this, he became invisible to those sitting  near him, and they went on talking as if he had gone. He wondered at this, and, fingering the ring, he turned the setting outwards again and became visible. So he experimented with the ring to test whether it indeed had this power—and it did. If he turned the setting inward, he became invisible; if he turned it outward, he became visible again. When he realized this, he at once arranged to become one of the messengers sent to report to the king. And when he arrived there, he seduced the king’s wife, attacked [b] the king with her help, killed him, and took over the kingdom.
Let’s suppose, then, that there were two such rings, one worn by a just and the other by an unjust person. Now, no one, it seems, would be so incorruptible that he would stay on the path of justice or stay away from other people’s property, when he could take whatever he wanted from the marketplace with impunity, go into people’s houses and have sex with anyone he wished, kill or release from prison anyone he wished, and do [c] all the other things that would make him like a god among humans. Rather his actions would be in no way different from those of an unjust person, and both would follow the same path. This, some would say, is a great proof that one is never just willingly but only when compelled to be. No one believes justice to be a good when it is kept private, since, wherever either person thinks he can do injustice with impunity, he does it. Indeed, every man believes that injustice is far more profitable to himself than justice. And any exponent of this argument will say he’s right, for someone [d] who didn’t want to do injustice, given this sort of opportunity, and who didn’t touch other people’s property would be thought wretched and stupid by everyone aware of the situation, though, of course, they’d praise him in public, deceiving each other for fear of suffering injustice. So much for my second topic.
As for the choice between the lives we’re discussing, we’ll be able to make a correct judgment about that only if we separate the most just and [e] make a correct judgment about that only if we separate the most just and the most unjust. Otherwise we won’t be able to do it. Here’s the separation I have in mind. We’ll subtract nothing from the injustice of an unjust person and nothing from the justice of a just one, but we’ll take each to be complete in his own way of life. First, therefore, we must suppose that an unjust person will act as clever craftsmen do: A first-rate captain or doctor, for example, knows the difference between what his craft can and  can’t do. He attempts the first but lets the second go by, and if he happens to slip, he can put things right. In the same way, an unjust person’s successful attempts at injustice must remain undetected, if he is to be fully unjust. Anyone who is caught should be thought inept, for the extreme of injustice is to be believed to be just without being just. And our completely unjust person must be given complete injustice; nothing may be subtracted from it. We must allow that, while doing the greatest injustice, he has nonetheless provided himself with the greatest reputation for justice. If he happens to make a slip, he must be able to put it right. If any of his [b] unjust activities should be discovered, he must be able to speak persuasively or to use force. And if force is needed, he must have the help of courage and strength and of the substantial wealth and friends with which he has provided himself.
Having hypothesized such a person, let’s now in our argument put beside him a just man, who is simple and noble and who, as Aeschylus says, doesn’t want to be believed to be good but to be so.1 We must take [c] away his reputation, for a reputation for justice would bring him honor and rewards, so that it wouldn’t be clear whether he is just for the sake of justice itself or for the sake of those honors and rewards. We must strip him of everything except justice and make his situation the opposite of an unjust person’s. Though he does no injustice, he must have the greatest reputation for it, so that he can be tested as regards justice unsoftened by his bad reputation and its effects. Let him stay like that unchanged until [d] he dies—just, but all his life believed to be unjust. In this way, both will reach the extremes, the one of justice and the other of injustice, and we’ll be able to judge which of them is happier.
Whew! Glaucon, I said, how vigorously you’ve scoured each of the men for our competition, just as you would a pair of statues for an art competition.
As I say, here we have an excellent example of what in contemporary philosophy would be called a thought experiment. Extract and articulate as clearly as you can the Ring of Gyges Thought Experiment. As with any good experiment, it serves as a demonstration. What here should we take as being demonstrated? Given the claims made posing it by both Glaucon and, later, Adeimantus, what does the thought experiment betray about the discussants' understanding of justice at this stage of the dialogue? How does the thought experiment's formulation constrain the range of possible responses to it?
Fourth Question: The City and the Soul
Socrates responds to Glaucon and Adeimantus' challenge by proposing a strategy grounded in an important analogy:
That’s well said in my opinion, for you must indeed be affected by the divine if you’re not convinced that injustice is better than justice and yet can speak on its behalf as you have done. And I believe that you really are unconvinced by your own words. I infer this from the way you live, [b] for if I had only your words to go on, I wouldn’t trust you. The more I trust you, however, the more I’m at a loss as to what to do. I don’t see how I can be of help. Indeed, I believe I’m incapable of it. And here’s my evidence. I thought what I said to Thrasymachus showed that justice is better than injustice, but you won’t accept it from me. On the other hand, I don’t see how I can refuse my help, for I fear that it may even be impious to have breath in one’s body and the ability to speak and yet to stand idly by and not defend justice when it is being prosecuted. So the best course [c] is to give justice any assistance I can.
Glaucon and the others begged me not to abandon the argument but to help in every way to track down what justice and injustice are and what the truth about their benefits is. So I told them what I had in mind: The investigation we’re undertaking is not an easy one but requires keen eyesight. Therefore, since we aren’t clever people, we should adopt the [d] method of investigation that we’d use if, lacking keen eyesight, we were told to read small letters from a distance and then noticed that the same letters existed elsewhere in a larger size and on a larger surface. We’d consider it a godsend, I think, to be allowed to read the larger ones first and then to examine the smaller ones, to see whether they really are the same.
That’s certainly true, said Adeimantus, but how is this case similar to [e] our investigation of justice?
I’ll tell you. We say, don’t we, that there is the justice of a single man and also the justice of a whole city?
And a city is larger than a single man?
It is larger.
Perhaps, then, there is more justice in the larger thing, and it will be easier to learn what it is. So, if you’re willing, let’s first find out what sort  of thing justice is in a city and afterwards look for it in the individual, observing the ways in which the smaller is similar to the larger.
That seems fine to me.
If we could watch a city coming to be in theory, wouldn’t we also see its justice coming to be, and its injustice as well?
And when that process is completed, we can hope to find what we are looking for more easily?
[b] Of course.
Plato often employs such analogical strategies to press inquiry, yet although it may be that we feel we have a better grasp of the nature of the concept examined under analogy, the argumentative strategy is predicated on the analogy holding true enough that inferences drawn by analogy hold. Set out and explain the inferences being drawn between state and soul. Are these inferences licit, or are there important disanalogous features which block them? Use examples to explain your answer either way. What other argumentative strategies might have been employed to discern the essential nature of a moral virtue like justice? Has justice an objective, essential nature, as Plato assumes, or has it no such independent nature, as the sophist Protagoras suggested when he claimed,
Of all things the measure is man, of the things that are, that [or "how"] they are, and of things that are not, that [or "how"] they are not. (DK80b1)