Book II aims to further outline this complicated question, and hopefully lead them closer to an answer. Glaucon isnt satisfied by the previous explanations on the nature of justice and injustice. To satisfy his hunger for knowledge, he proposes a challenge to Socrates. Glaucon wants Socrates to explain how justice could be intrinsically good, or, in other words, how justice could be welcomed for its own sake, such as we welcome joy for its own sake. Glaucon expresses this challenge by defining to Socrates the three kinds of goods.
Intrinsic goods, he says, are those that are welcomed for their own sake, and not for what rewards could possibly come from them. Mixed goods are those that we welcome for their own sake, but also for what possible rewards could come from them. Instrumental goods are those that we only welcome for the rewards that come from them. Glaucon believes that Socrates could prove that justice is a mixed good by proving exactly how it is instrinsic. Glaucon, in an attempt to reiterate Thrasymachuss argument in Book I, goes on to present a three-part argument proving that injustice is better than justice.
In his first point, Glaucon explains the common conception of justice and its origins. Essentially, the natural origin of justice comes from the fact that people like doing injustice, but it is worse to endure justice. Because of this, everyone comes to an agreement not to do injustice so they dont have to suffer it. Since this story of the origins proves that justice is purely instrumental, if one was to accept this story they would also accept the next two points and, ultimately, accept Thrasymachuss position. Glaucons second point is that justice is purely instrumental.
He illustrates this point by using the example of the ring of Gyges. If two people both had the power to do whatever they wanted with no repercussions, as the ring of Gyges would allow them to do, Glaucon thinks that both would end up following the path of the unjust, and be better and happier for it. The just person wearing the ring could do unjust acts but still keep his reputation for justice. Glaucon says, No one believes justice to be a good when it is kept private, since, wherever either person thinks he can do injustice with imputiny, he does it (360c).
This leads to his third point. Glaucon thinks that the completely unjust person is much happier than the just person. The ideal unjust person is able to attain everything they could ever want and need, while being honored and praised by those around him for seeming like a just person. Oppositely, the ideal just person is just but doesnt care about seeming just, thus leading others to believe he is unjust, possibly for his whole life. When the two are compared side by side, it is clear how the unjust person is happy, but not how the just person is happy.
Socrates must take certain steps to disprove Thrasymachuss position on justice. Because the origin story is the key point of Glaucons argument, Socrates must show why this origin story seems right but is not. In order to do this, he needs to present a new origin story that shows exactly how justice is instrinsic. Socrates decides that in order to do this he must start by looking at justice in a large sense, then narrow it down to a smaller sense.
To outline this process, he states, We say, dont 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? Perhaps, then there is more justice in the larger thing, and it will be easier to learn what it is. So, if youre willing, lets 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 (368d-369a).
In order to look for justice in the city, Socrates must map out an ideal and perfectly just city. By showing the exact specifications of this perfect city, including the classes, functions, and virtues of the people, Socrates can undermine Thrasymachuss argument and answer Glaucons challenge. To define justice and to answer the many questions surrounding justice is one of the main topics of Platos The Republic, and Socrates sets out to find these answers for the remainder of the book.