Plato's Republic, Book III

I am going to defend Socrates’ attempted representation of justice through an ideal city.

Because there is no “teacher of justice,” we can only assume that justice doesn’t derive from man. Thus, we cannot look to men to find justice itself. There must be an outside source. We can only look to men in hopes of finding illustrations of justice. So, if we are, in fact, trying to find justice, we must look to its source, not its outcomes. We must see it for what it is when it’s not mixed up in human emotion, varied perceptions, evil, etc. The city that Socrates is building is being built on justice and justice alone—not its interactions with human nature or city life. Because we are on the pursuit for something much grander than ourselves, we have to aim at something higher than our natural scope. We must aim at justice, in its purest form, so that we have something to look to. By establishing justice alone, we gain access to a model of the way things should be (though they are not). Though it's impossible to have a perfectly just society (given human nature), and he knows this, Socrates is attempting to make it possible to aim for one.

Also, when CIA agents are trained to detect counterfeit money, they do not study all the ways that a dollar bill can be counterfeit. Rather, they study the real dollar bill so diligently that they know the second they touch or see a counterfeit. This is a response to the question raised about the possiblity of knowing justice when we have not studied injustice. If we come to know justice well, we will eventually know the second we come across an area or action that is absent of justice.