  In Book Four Socrates says that the just person is wise and thus knows what is good for him, but he does not say anything about what knowledge or the good is. In Books Five through Seven he clearly addresses these issues and fills out his account of virtue. He shows, in sum, that one is virtuous if and only if one is a philosopher, for he adds to Book Four’s insistence that virtue requires knowledge the new claim that only philosophers have knowledge (esp. 474b–480a). His account also opens the possibility that knowledge of the good provides the crucial link between psychological justice and just actions.