Rebecca Newberger Goldstein, from Plato at the Googleplex: Why Philosophy Won't Go Away:
Socrates was probably the first to identify aretē with what is — in terms of a person’s moral makeup or moral character — analogous to the health in that person’s body. A person doesn’t have to be recognized as healthy in order to be healthy, and so it is with Plato’s Socrates’ aretē. In the myth of the Ring of Gyges, presented in the Republic, Plato argues that even if a person could get away with all manner of wrongdoing while maintaining a good reputation because of a magic ring that renders him invisible, still he should not do any of these awful things, since by destroying his aretē the man will destroy himself. Aretē then is entirely independent of social regard. And in the Gorgias, Socrates is presented as asserting something so radical that his hearers think it has to be a joke. He would, he says, rather be treated unjustly than treat others unjustly (469c). But if aretē is conceived of as analogous to the health of the body, then Socrates’ statement is hardly absurd. The injustice that we do involves us far more intimately than the injustice that we suffer.