Much has been written about the inadequacy of the city-soul analogy in establishing what justice is, and further about how Plato fails to adequately connect his vision of justice to the conventional one and so is unable to address the original challenge. I mean to show that the city-soul analogy is in fact compelling, or at least that is it sufficiently adequate to allow us to move on to a discussion of how Platonic justice compares to conventional justice. At that point I will attempt to show that Platonic justice is relevant to the challenge posed to Socrates, and that despite objections to the contrary the Platonic and conventional views are sufficiently aligned to allow Socrates to conclude that he has shown that it is better to be just than unjust.
Vlastos, and others, argue that describing the city as just is simply a generalization about its members, and so the city is not just in the same way that a person is just. I wish to argue, as Wilson does, that there are other grounds for Plato to attribute justice to the city. As Wilson puts it, “[Plato’s] central question is not the analytical philosopher’s question ‘What does ‘justice’ mean?’, but the substantial question ‘What is justice?’”. Thus, it is wrong to criticize the Republic as one would criticize a formal argument towards a definition, as Plato is actually searching for what justice actually is. Wilson’s doctor metaphor is helpful: Plato is investigating the nature of justice just like doctors inquire into the nature of a disease. Doctors notice a variety of systems and become convinced they are caused by a single underlying condition, to which they attribute a name. Eventually someone discovers this underlying condition and the initial assumption is justified. In this way, something like polio can be understood, not by investigating the meaning of the word polio but by looking at possible examples of it and by studying its nature. Plato, Wilson argues, is doing something similar with justice.
What is it that makes the connection between justice in a city and justice in an individual plausible? To avoid the objections to the city-soul analogy, it is important that justice and other virtues not be found in the city only in a derivative way, that is, only as a generalization about the city’s members. Luckily, Plato is able to avoid this method. Wilson proposes that Plato does so by equating the structural features of the soul and of the city. Then, when some virtue V is indicated by certain characteristics C of a city, and by those same characteristics C of a soul, and that after inquiry we find that the structural feature of both the city and the soul that gives rise to C is the same structural feature S, then like the doctor we can conclude with certainty that V is S. Whether or not Plato successfully equates the structural features of the soul and the city is a large question, but he certainly thinks that he does. I do not mean to go into a discussion of this separate question here, but just wish to have shown that the use of the city-soul analogy is perfectly reasonable, since it provides an adequate method for investigating the nature of justice.
Another potential problem Plato faces is the connection of Platonic justice to conventional views of justice. The original challenge is to show that it is better for a man to be just than unjust, regardless of anything else. This question would certainly not be sufficiently answered by inventing a new definition of justice that was unconnected from conventional views of justice, for example if the new definition permitted acts that commonly would be considered unjust; it is necessary for Plato to demonstrate a connection between Platonic and conventional justice. This issue comes most pointedly from Sachs, who asserts that Plato has to prove two...