Aristotle, Nicomachean Ethics
H. Rackham, Ed.

our demand furnishes a sufficiently accurate common measure for practical purposes. [15] There must therefore be some one standard, and this accepted by agreement （which is why it is called nomisma, customary currency）; for such a standard makes all things commensurable, since all things can be measured by money. Let A be a house, B ten minae and C a bedstead. Then A=B/2 （supposing the house to be worth, or equal to, five minae）, and C （the bedstead） =B/10; it is now clear how many bedsteads are equal to one house, namely five. [16] It is clear that before money existed this is how the rate of exchange was actually stated—five beds for a house—since there is no real difference between that and the price of five beds for a house. [17]

We have now stated what Justice and Injustice are in principle. From the definition given, it is plain that just conduct is a mean between doing and suffering injustice, for the former is to have too much and the latter to have too little. And Justice is a mode of observing the mean, though not in the same way as the other virtues are, but because it is related to a mean, while Injustice is related to the extremes.