Aristotle opens with "perplexities": whether a person can suffer injustice (or justice) voluntarily, or if everyone who suffers something unjust suffers injustice (same for justice).
Also, is it possible to do an injustice to yourself?
Back to the first question: can a person suffer injustice voluntarily—say, by bad behavior on his part—by someone who is also acting voluntarily?
Aristotle wonders if we shouldn't add another dimension to his definition of injustice: that it has to be done voluntarily and against the other person's wishes.
He thinks it is a good addendum, since no one really wishes to be harmed, even if he acts self-destructively.
He concludes that it's not up to the person suffering to determine whether or not he's been a victim of injustice.
And so Aristotle answers his own questions: suffering injustice cannot be voluntary, since no one wishes to be harmed.
Also, if an act is to be unjust, it has to have been voluntarily committed by a second party.
Two more picky details to consider: 1) Can a person who gives someone more than he deserves be unjust; 2) Or is it the person who receives more than he should the unjust one?
Aristotle answers these questions by engaging the earlier question about whether we can be unjust to ourselves.
He says that if giving more of our own goods than is deserved is unjust, then we are unjust to ourselves.
But since an unjust act must be against the person's wish—and the giver gives voluntarily—there can be no self-inflicted injustice.
Neither is the receiver unjust, though it's technically an unjust thing to receive more than you're worth.
And yet, the giver does do an unjust thing by distributing more than what a person deserves. As you can see, Aristotle does lots of fence-sitting here.
Ultimately, it depends on the giver's intentions. If he has made an error in judgment, then he himself is not an unjust person, though the act of giving too much is unjust.
If he has judged correctly, he's perhaps giving too much in the hopes of gaining more than his fair share of honor. And that's somehow bad.
One thing is really clear from all this hemming and hawing: doing the just thing isn't as easy as it seems.
It's not simply choosing the right thing to do. We also have to have the right intention.
It's also difficult to discern between the just and unjust things, because the law tells us how to behave.
But as Aristotle says, the law is merely the tip of the justice iceberg.
Aristotle makes a final observation: justice is human.
Wherever there can be excess or deficiency of good things, that's where the principles of justice apply. (Hint: not with the gods).