Justice for an Individual in the Republic

Versions of justice in Book I and II:
Many conventional views of justice, today and in Plato's time, are "act-centered." All of the above theories of justive are act-centered.
Plato's theory of justice appears to be agent-centered (justice for an individual is harmony of the soul), which means that acts are only derivatively just.

Challenges to Plato (from David Sachs' in 'A Fallacy in Plato's Republic' (Phil. Rev. 1963, 141-58)):
  1. Unless Plato can show that his concept of justice is identical, or at least close enough, to conventional justice, then the Republic is just irrelevant when it comes to justice.
  2. The motive which Plato gives to people to be just is self-centered, and hence he is advocating rational egotism, which is neither praiseworthy nor a good reason to be just.
Initial Response to #1: Whether or not Plato's concept of justice is our conventional concept of justice, the Republic might say important and interesting things about what it is to be an excellent human. If those things turn out to be different from or even incompatible with conventional justice, so much the worse for conventional justice, which may be a concept similar to the theory of ether (i.e. a demonstrably false theory that should be replaced by a better one). The initial explorations of what justice is in Book I are just that, initial. They are fully superseded by what follows.

Initial response to #2: Plato is responding to Glaucon's challenge: to show why a self-centered person should want to be just. That he provides the self-centered person with self-centered reasons to be just is similar to Diodotus's Speech in Thucydides' Mytilenean debate (the Athenians should spare the Mytileneans because it is in their interest). As such, his reasons are tailored to an audience and do not preclude there being other reasons for being just.

An expanded response: if you have time, see my summary of Norman Dahl's article 'Plato's Defense of Justice':

Dahl has a good way of formulating the three classes of goods from 357-8:

1) Goods which are good "in virtue of what is due to those goods themselves, and not for what arises from them together with circumstances that normally accompany them."

2) goods which are good "both in virtue of what is due to those goods themselves and for what arises from them together with circumstances that normally accompany them."

3) goods which are burdensome in themselves but are good for what arises from them together with circumstances that normally accompany them.

That formulation makes it clear that Plato can claim that justice is good for its own sake and for the sake of happiness as long as happiness is internal to the agent. That is because " circumstances that normally accompany them" can be taken to refer to circumstances external to the agent (note that that leaves a gray area insofar as the distinction between agent and circumstances may be unclear).

Plato wants to show that justice belongs to class 2, but to do that, he has to show that justice is worth choosing even if one gets the usual rewards and reputation of INjustice, and even if the unjust person gets the rewards and reputation for justice.

VIRTUE and EUDAEMONIA
(The following are meant to be analytic statements: in other words, they explain the contents of the concepts happiness and virtue rather than offer arguments that should make us accept those concepts)

For Plato, virtue (arete) belongs to a thing that has a characteristic function (ergon): virtue is the quality that makes that thing perform its function well or perhaps the quality of performing that function well. So human virtue makes humans perform characteristically human activity (or activities) well. A 'characteristic activity' is one that a given thing alone does, or that the given thing does better than other things do it.

What that means is that virtue enables a human to live a good life, a life of eudaemonia.

Virtue is supposed to MAKE one act well and BE well off (as Thrasymachus' claim that injustice is a virtue because it makes one well off shows (348-9)).

Plato is 'internalist': full understanding of virtue provides one with a MOTIVE to be virtuous. I.e. if you know what virtue is, you cannot help but want to be virtuous: it has its own motivating force.

So Plato wants to give people a MOTIVE to be just and to claim that just people are BETTER OFF.