Book 5
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. Also, Justice is that quality in virtue of which a man is said to be disposed to do by deliberate choice that which is just, and, when distributing things between himself and another, or between two others, not to give too much to himself and too little to his neighbor of what is desirable, and too little to himself and too much to his neighbor of what is harmful, but to each what is proportionately equal; and similarly when he is distributing between two other persons.
