These notes are built primarily from a selective summary of Chapter 4 in Richard Kraut's Aristotle. Kraut gives many references in Aristotle, and I encourage you to refer to Kraut if you are interested in a particular bit of the following summary.
For Aristotle, there are two senses of justice:
"Lawfulness" may make it
sound as though Aristotle thinks that merely obeying whatever those in
power say is always just (sound like Thrasymachus?), that obeying the
letter of the written laws
is all there is to justice, and as though all laws are always just.
That is not really the case:
Aristotle rejects Plato because he holds that justice is necessarily exhibited by action and involves interpersonal relations. Plato held that justice is an intrapersonal phenomenon. Aristotle held that one must involve oneself in one's city's affairs to be fully just, whereas Plato held that philosophers in most cities must withdraw from public affairs in order to be just. Aristotle held that improving imperfect systems was just. Plato held that such imperfect systems would only corrupt and victimize those who engaged in them.
Aristotle's method of defining justice is by means of opposites: he determines what injustice consists in, and argues that its opposite is just. That procedure works well with certain sorts of "opposites," and seems to work with justice.
The "equality" part has to do with not grasping for more than what
is fair. The just person wants their
fair share, not more, not less, and so that fits with
Aristotle's contention that virtues are means between extremes (think of
Plato's argument that the virtuous optimize whereas the vicious
maximize as used against Thrasymachus).
4.3
Aristotle thought that unjust laws exist (1282a41). In fact, a whole community could be mistaken in their concept of their ultimate goal, well-being (Sparta is a common example used by A., the details of which are not important here).
So justice is the intellectual and emotional skill which brings it about that a community possesses a stable system of laws, rules, and customs that further the attainment of its ultimate goal, well-being (happiness). To be just involves possessing all the other virtues as well (1129b25). The just person will be skilled in resolving disputes, distributing goods, and meting out punishments (reciprocal, distributive, and corrective justice).
Notice that like Plato's justice, Aristotle's
justice does not give
us directions for how to act in particular situations. It gives
us
general guidelines: Aristotle was explicitly clear that no science or
area of knowledge should have conclusions that are more precise than
the nature of the subject matter allows.
4.4
Laws are too general. The juror must call on justice to help decide particular cases equitably and to correct the deficiencies that even a well-crafted law will exhibit.
So "lawfulness" in fact involves not obeying the letter of the law, right or wrong, but rather it is a laborious process that involves applying the law. But the juror is not simply to twist the law to fit his or her own views. The juror must take on the point of view of the legislator and follow their intent!
An individual is not at all justified in disobeying an unjust law. That individual should be punished, but the individual should also try to change the law within the system. Lawlessness is worse than the inevitable deficiencies of any particular system of laws. Think of what Aristotle would say about Antigone and Creon: he would certainly be more sympathetic to Creon than most moderns are.
4.5
All legal systems aim at the good of the community: they are not pointless regulations deserving obedience simply because they exist.
Obeying an unjust law is still worth doing because the whole of the system is important, and legal systems, imperfect as they almost always are, could not exist unless people were willing to obey imperfect laws. Having a reliable system for resolving disputes is valuable, simply because it is reliable. Being relatively certain in one's expectations is also valuable, simply because one can be relatively certain. Thus lawfulness promotes the well being of the community at large even in cases where laws are imperfect.
Even though Greek cities were exclusive about citizenship, much of their laws were formulated for the good of the entire community. That seed of an ideal is important!
4.6
Justice is unique among the virtues in that the other virtues can be done on one's own in a private capacity, but the other virtues become justice when they are done for the good of the community. E.g., refraining from eating the whole thing is temperate, but if there is more than enough to go around, it is just temperate for oneself. If, however, there is not enough, or just enough, to go around, then it is also for the sake of the community, and so temperance can be just. In other words, an action that fits under any other virtue can be done in isolation, but an action only becomes just when it involves the good of some community.
4.7
Aristotle distinguishes between various kinds of justice. Political justice affects only one's fellow-citizens, and so is only a part of justice. Household justice involves husband and wife, son and wife, etc. Aristotle thinks that a father's family members are like parts of himself, and so he will be just to them.
Aristotle further distinguishes between natural justice and legal justice.
For example, it is legally just that one sacrifice two goats rather
than one, because that is the norm of one's community, but it is not
a matter of natural justice to do so.
A legally just thing is
4.17 Aristotle versus Plato
If Aristotle is right, then the philosophers who withdraw and refuse
to be involved are perhaps guilty of injustice by being in a position
to improve civic life but failing to do so (be careful here, however).
Plato required the account of the ideal city to tease out his concept of justice. Aristotle elucidates his account prior to any discussion of politics.
Plato's philosopher is not of any obvious use outside of the ideal city. Aristotle's just person (whether or not he or she is actually a real philosopher) will be of benefit to his or her city because of his or her justice.
4.18 Socrates and Aristotle
As a man, rather than as a thinker, Socrates too withdrew from public life. He preferred to speak with individuals in a private capacity. In a significant sense, Socrates was not just according to Aristotle. He did not benefit Athens as he could have.
Socrates was, however, a man with tremendous respect for law.
The key to the theoretical difference between Aristotle and Socrates' conception of justice is that Socrates held that rationality is all there is to justice. It is a matter of knowledge. If you know what is right, you do it, period. Aristotle, however, held that habit was as important as knowledge, hence he would set up an elaborate system of education in his ideal city, and he held that much of virtue is about habituation. The very term "ethics" is from a Greek word that means "habits," among other things. Thus like Plato, Aristotle held that habituation is as important as knowledge for virtue. If you are virtuous based on knowledge, your virtue might be vulnerable to evil persuasion, but if you have been raised in such a way that your default is virtue, you will act virtuously out of habit (but you may not be fully virtuous): adding the rational components of virtue to your actions will make them fully virtuous.
So what of Aristotle himself? He was a foreigner with no citizen status in Athens. He was not publicly involved in the ways he requires that a politically just person be involved!