
Reason, spirit and desire. The city is likewise divided into three classes: the rulers, the auxiliaries and the producers. A person is of a specific class dependent on which part of their soul is predominant.
In order for the soul to be just, reason, spirit and desire must each perform its proper function and all work in harmony together. Reason must rule the soul, spirit must support and defend it, and desire must acquiesce to it. Similarly, in order for the city to be just, each class must perform its proper function and work in harmony with the other classes. Rulers must rule, auxiliaries must support and defend the rulers and the producers must obey the rulers.
( , Tue 3 Feb 2009, 22:30, archived)

as it seems to suggest that only the rulers can be just. Justice is the result of all parts of the soul performing well. Part of the function of the reasoning part of the soul is to have knowledge of truth. Since the proles are massive thickies, only the rulers (the philosopher-kings) can have the kowledge required to have optimally reasoning souls, and so they can be the only ones who are just.
( , Tue 3 Feb 2009, 22:38, archived)

i skipped over Plato in college, please don't think any less of me :/
( , Tue 3 Feb 2009, 22:39, archived)

that the producers and auxiliaries can have a kind of justice-by-proxy. Although they cannot know truth, as they aren't of the right class and so don't have the requisite skills and education, they can accept the wisdom of those who can know truth and allow themselves to be governed by the ruling classes.
And so Plato's account of a just city, where the producers and auxiliaries allow themselves to be ruled by the rulers, does also involve every individual being just.
( , Tue 3 Feb 2009, 22:42, archived)