Bloom’s interpretation follows from an understanding of Plato’s ideas about justice and just cities in The Republic, which is how the book demands to be read at first. Looking at The Republic as a work on justice, we first need to ask why justice has to be defended. As Thrasymachus makes clear, justice is not universally assumed to be beneficial. For as long as there has been ethical thought, there have been immoralists, people who think that it is better to look out for your own interest than to follow rules of right and wrong.

Traditionally, the Greek conception of justice came from poets like Hesiod, who in Works and Days presents justice as a certain set of acts that must be followed. The reason for being just, as presented by the traditional view, was consideration of reward and punishment: Zeus rewards those who are good and punishes those who are bad. In late fifth century Athens, this conception of divine reward and retribution had lost credibility. No one believed that the gods rewarded the just and punished the unjust. People could see that many unjust men flourished, and many of the just were left behind. In the sophisticated democracy that evolved in Athens, few were inclined to train their hopes on the afterlife. Justice became a matter of great controversy.

Leading the controversy were the Sophists, the general educators hired as tutors to the sons of the wealthy. The Sophists tended not to believe in objective truth, or objective standards of right and wrong. They regarded law and morality as conventions. The Sophist Antiphon, for example, openly declared that we ought to be unjust when being unjust is to our advantage.

Plato felt that he had to defend justice against these onslaughts. The Sophistic challenge is represented in The Republic by Thrasymachus, who declares that justice is nothing but the advantage of the stronger. Since this statement motivates the entire defense that is to follow, it deserves analysis. What exactly does Thrasymachus mean by claiming that justice is the advantage of the stronger? Who are the stronger? What is their advantage?

On the first reading, Thrasymachus’s claim boils down to the basic Sophistic moral notion that the norms and mores we consider just are conventions that hamper those who adhere to them and benefit those who flout them. Those who behave unjustly naturally gain power and become rulers and strong people in society. When stupid, weak people behave in accordance with justice, they are disadvantaged, and the strong are at an advantage. An alternate reading of Thrasymachus’s bold statement makes his claim seem more subtle. On this reading, put forward by C.D.C. Reeve, Thrasymachus is not merely making the usual assertion that the norms and mores of justice are conventions; he is further claiming that these mores and norms are conventions put in place by rulers to promote their own interests and to keep their subjects in a state of oppression.

This second reading is interesting because it challenges not only our conception of right and wrong, but Socrates’s usual way of finding truth. Socrates’s method of elenchus proceeds by building up knowledge out of people’s true beliefs. If Thrasymachus is right, then we do not have any true beliefs about justice. All we have are beliefs forced on us by rulers. In order to discover the truth about right and wrong, we must abandon the old method and start from scratch: building up knowledge without resting on traditional beliefs. In the next book, Plato abandons the method of elenchus. and begins the discussion from scratch.

Regardless of how we interpret Thrasymachus’s statement, the challenge to Socrates is the same: he must prove that justice is something good and desirable, that it is more than convention, that it is connected to objective standards of morality, and that it is in our interest to adhere to it.