Plato, Book IV of The Republic.

In Book IV of The Republic, Socrates has gone on a bit of a digression. He has done this for the sake of proving his argument as to what constitutes the ideal city. In order to do this, Socrates feels that he and Glaucon must evaluate the individual citizen, specifically the mind and soul of the individual citizen. A city in itself is nothing more than a city, it is the individuals within it who …

…not struggle against it; justice is brought about when all three parts carry out their own responsibilities specifically without interfering with the duties of any of the other parts. In a final attempt to persuade Glaucon of his opinion on the matter of the soul and its components, Socrates' reasoning appears to be satisfying. Every claim that he makes is backed up with examples and analogies that are more than capable of proving his arguments.