Let's try again.
When talking about the relationship between justice and anything else, we should try to define what is meant by "justice." This is not easy!
Plato understands individual justice on analogy with justice “writ large” in the state, but he views the state, or republic, as a kind of organism or beehive, and the justice of individuals is not thought of as primarily involving conformity to just institutions and laws. Rather, the just individual is someone whose soul is guided by a vision of the Good, someone in whom reason governs passion and ambition through such a vision. When, but only when, this is the case, is the soul harmonious, strong, beautiful, and healthy, and individual justice precisely consists in such a state of the soul. Actions are then just if they sustain or are consonant with such harmony.Let's talk about where justice is properly located. Both of these philosophers are treating justice as an individual phenomenon that has links to a social or political phenomenon. Where is justice to be found?
Such a conception of individual justice is virtue ethical because it ties justice (acting justly) to an internal state of the person rather than to (adherence to) social norms or to good consequences; but Plato's view is also quite radical because it at least initially leaves it an open question whether the just individual refrains from such socially proscribed actions as lying, killing, and stealing. Plato eventually seeks to show that someone with a healthy, harmonious soul wouldn't lie, kill, or steal, but most commentators consider his argument to that effect to be highly deficient.
Aristotle is generally regarded as a virtue ethicist par excellence, but his account of justice as a virtue is less purely virtue ethical than Plato's because it anchors individual justice in situational factors that are largely external to the just individual. Situations and communities are just, according to Aristotle, when individuals receive benefits according to their merits, or virtue: those most virtuous should receive more of whatever goods society is in a position to distribute (exemptions from various burdens or evils counting as goods). This is what we would today call a desert-based conception of social justice; and Aristotle treats the virtue of individual justice as a matter of being disposed to properly respect and promote just social arrangements. An individual who seeks more than her fair share of various goods has the vice of greediness (pleonexia), and a just individual is one who has rational insight into her own merits in various situations and who habitually (and without having to make heroic efforts to control contrary impulses) takes no more than what she merits, no more than her fair share of good things.
Another discussion last week involved an analogy to water: it isn't reducible to the oxygen and hydrogen that are its parts, I said, because water has properties of its own that the components do not have. The relationship creates a new thing that is just as real as the components (and even hydrogen and oxygen are, after all, nothing more than relationships of sub-atomic particles, which are themselves only relationships of another kind). Properties that come to be realized at higher levels of organization are called "emergent properties," and we can say that a property belongs to the level of its emergence -- wetness, so to speak, belongs to water rather than to oxygen or hydrogen.
So where does justice emerge? It seems that on Plato's account it emerges in the individual, but on Aristotle's it does not emerge until there are multiple individuals in relationship to one another. For Plato, it would be possible to speak of an individual as just because he was guided by the Good, and so he could be just while dining alone -- he would be just, in a sense, by being moderate with his food so as to maximize his capacities. For Aristotle, justice is about not taking more than what is fair given your own value and virtues. Moderation is a virtue, and it is related to justice because it is what allows you to resist the temptations that might cause you to be unjust.
Either way, justice is a property of pre-political levels. Either it emerges in the individual soul, or it emerges at the level of first relationships -- family relationships, naturally, because our first relationships are the relationships with those who bring us into the world and sustain us. And indeed Aristotle will talk, in the Politics, about how political unions form out of the family unions that are our first society.
Justice is therefore not a property that belongs to the law. It is a pre-political virtue. Why, then, do we associate it with the law?
It seems that we have less trouble being just to those we love. On Plato's account, this makes sense: if we are guided by the Good, by definition we desire the good for those we love. On Aristotle's account it is a bit harder, until you realize that he regards friendship also in terms of virtue. It is possible to have lesser species of friendship that are just for useful things, or because they are pleasant, but a true friendship is brought about by the admiration you have for the virtues of another. It is therefore easy not to wish to take more than is fair from those you admire, because you want them to think well of you in return. Likewise, you naturally desire the good for those you befriend, for if they did not obtain things that were good for them, they would cease to be.
When families or other pre-political groups try to assemble themselves into larger groups, however, it is not as easy to be fair to each other. It is, in fact, more natural to continue to favor those whom you love -- either as family or as friends -- and to try to obtain extra advantages for them (or yourself).
Yet the reason we want a larger society is so that we can obtain some kind of benefit from others outside our intimate circles. They do not wish to be exploited, nor do we wish to be exploited by them. So we create rules, agreements, that should govern our interactions to make sure that they are fair.
Still disputes arise. One group claims that the other group didn't adhere to the rules, or broke an agreement. If this is not to lead to fighting and a breakdown of the society (and its benefits), an accord must be made between the parties. Sometimes the parties are virtuous enough to work it out between themselves. Often, though, some respected third party must be brought in to solve the problem.
If this is done by negotiation, and the third party is respected by both, no state is necessary even here. But if it is done by force, and the adjudicating party is not followed by will but because it has the capacity to compel obedience, then you have a state and laws.
So it seems that justice in the law lies in having an institution that is capable of forcing us to treat our fellow subjects in the same way that we would treat those we love, i.e., our friends and family. It forces us to keep the arrangements we made, and requires us to make them in such ways that they are not exploitative. If the law does that, it is performing the function for which the rules were wanted, and thus enabling the society to function.
Yet this seems to be improper. There are many ways in which our intimate connections are rightly privileged by us, especially if Aristotle is right about the nature of justice. If justice is getting what you deserve, who deserves more from me than my father? If I treat him the same way that I treat another, I am being unjust, not just.
This seems to me to indicate that there is a severe tension when we look for justice in the law. The kind of 'justice' it can achieve is only justice by analogy, and itself out of order with the true virtue of justice. True justice lies in the soul, either in a vision of the good or in the sense of love that belongs to those you who most deserve it from you.
That is not to say that the law should make no attempt at this justice-by-analogy. However, it is to say that true justice is impossible for the law, or for the state. If justice is desired, and it is surely desirable, the state and the law must be carefully constrained to their proper and limited role. We should use the state or the law no more than absolutely necessary to enable the benefits of a larger, political society. Nor should the state be allowed to transgress into the intimate spaces where true justice is possible, because the best it can achieve is a mere shadow of true justice. People should be free to depart from such bonds if they fail to be just, but the power to sever or re-order such bonds ought to be located only in the individual, not in the state.