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our demand furnishes a sufficiently accurate common measure for practical purposes.  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.  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.  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.