Aristotle, Nicomachean Ethics
H. Rackham, Ed.

The other kind is that which supplies a corrective principle in private transactions.2. [13] This Corrective Justice again has two sub-divisions, corresponding to the two classes of private transactions, those which are voluntary and those which are involuntary.¹ Examples of voluntary transactions are selling, buying, lending at interest, pledging, lending without interest, depositing, letting for hire; these transactions being termed voluntary because they are voluntarily entered upon.² Of involuntary transactions some are furtive, for instance, theft, adultery, poisoning, procuring, enticement of slaves, assassination, false witness; others are violent, for instance, assault, imprisonment, murder, robbery with violence, maiming, abusive language, contumelious treatment.3.

Now since an unjust man is one who is unfair, and the unjust is the unequal, it is clear that corresponding to the unequal there is a mean, namely that which is equal; 3. [2] for every action admitting of more and less admits of the equal also. 3. [3] If then the unjust is the unequal, the just is the equal—a view that commends itself to all without proof; and since the equal is a mean, the just will be a sort of mean too. 3. [4] Again, equality involves two terms at least. It accordingly follows not only （a） that the just is a mean and equal [and relative to something and just for certain persons³], but also （b） that, as a mean, it implies certain extremes between which it lies, namely the more and the less; （c） that, as equal, it implies two shares that are equal; and （d） that, as just, it implies certain persons for whom it is just. 3. [5] It follows therefore that justice involves at least four terms, namely, two persons