 If Plato went through a period of open-ended experimentation, this stage was definitely over when he wrote the Republic, the central work of his middle years. Because of the Republic’s importance a more detailed account will be necessary, in order to explain the ethical principles set forth in that work, for the principles are closely intertwined with political, psychological, and metaphysical conceptions. That the work represents a major change in Plato’s thinking is indicated already by the dialogue’s setting. The aporetic controversy about justice in the first book is set off quite sharply against the cooperative discussion that is to follow in the remaining nine books. Like the Gorgias, the first book of the Republic presents three interlocutors who defend, with increasing vigor, their notion of justice against Socrates’ elenchos. Of these disputes, the altercation with the sophist Thrasymachus has received a lot of attention, because he defends the provocative thesis that natural justice is the right of the stronger, and that conventional justice is at best high-minded foolishness. The arguments employed by Socrates at the various turns of the discussion will not be presented here. Though they reduce Thrasymachus to angry silence, they are not above criticism. Socrates himself expresses dissatisfaction with the result of this discussion R. 354c: “As far as I am concerned, the result is that I know nothing, for when I don’t know what justice is, I’ll hardly know whether it is a kind of virtue or not, or whether a person who has it is happy or unhappy.” But for once, the confession of aporia is not the end of the discussion. Two members of the audience, Plato’s brothers Glaucon and Adeimantus, challenge Socrates: Perhaps Thrasymachus has defended his case badly, but if Socrates wants to convince his audience, he must do better than that. The brothers demand a positive account of what justice is, and of what it does to the soul of its possessor.