The Republic of Plato

After reading Book II in "The Republic of Plato" I have many questions. I understand what it is that Glaucon and his brother are trying to say. Justice is a concept of a group of people that is created by fear of injustice. It's a necessary evil in their opinion. You do not commit unjust acts towards others so that others will not commit them towards you. It's like an early version of the golden rule. I don't however understand his definition of a just or unjust man. In Glaucon's story about the ring that makes you invisible he is trying to show that even a just man will act unjustly if given the chance to do so with impunity. By committing unjust acts he shows himself to be unjust. I don't really agree with Glaucon, justice is something inherent to