Perhaps the most astounding feature of The Republic is the range of issues that it covers. Socrates begins with an ethical question, but his discussion ranges over politics, knowledge, the nature of reality, education, mathematics, art and psychology. But the dialogue is not a hotchpotch of loosely related topics - Socrates has a clear conception of how these issues intermesh. In particular, he argues, we cannot understand the value of justice - personal or political - unless we understand what goodness is in reality, and how we come to know it.

So how does Socrates’ answer Glaucon’s question? He begins with an account of the soul. The soul, he suggests, contains three elements: desire (desire for food, sex and so on); spirit (in the sense of spiritedness or feistiness); and reason. In the healthy soul, reason will rule desire, with the assistance of spirit. Someone who has a healthy soul will be just inside and out: just on the inside, because it is right that reason should rule; and just on the outside, because someone who is ruled by reason will not be the sort of person to rob and cheat. Finally, Socrates suggests, we should want to have a healthy soul, just as we want to have a healthy body. We would not want to be like Gyges - a chaotic individual, ruled by his desires.

Is it true that someone who is ruled by reason will act justly towards others? We can easily imagine a rational villain. But Socrates has an answer to this objection. According to Socrates, the rational part of the soul loves truth; it aims at, above all, knowledge of the good. The person who is ruled by reason will be a philosopher who seeks knowledge of the good. Socrates also assumes that to know the good is to prize it. So the philosopher will understand how to act well and justly, and will wish to do so.

What does this imply about trust? Not much, it seems. We can trust the philosopher not to rob or cheat. But not many people are philosophers in Socrates’ sense. For security, we will still have to rely on laws, and that presupposes a political system.

In The Republic, Socrates offers an account of the just state, alongside his account of the just soul. Whether the account of the state is intended as a piece of political philosophy or only as an analogy for the soul is highly controversial. But it has certainly been interpreted as a blueprint for utopia by other thinkers.

Socrates suggests that the state ...