And when they
exchange their products they must reduce them to the form of a proportion, otherwise one
of the two extremes will have both the excesses1
whereas when they have their own,2
they then are equal, and can form an association together,
because equality in this sense can be established in their case （farmer A, food
C, shoemaker B, shoemaker's product equalized D3
）; whereas if it were impossible for
reciprocal proportion to be effected in this way, there could be no association between
That it is demand which, by serving as a single standard, holds such an association
together, is shown by the fact that, when there is no demand for mutual service on the
part of both or at least of one of the parties, no exchange takes place between them
[as when someone needs something that one has oneself, for instance the state
offering a license to export corn in exchange for wine].4
This inequality of demand has therefore to be equalized.
Now money serves us as a guarantee of exchange in the future: supposing we need nothing
at the moment, it ensures that exchange shall be possible when a need arises, for it meets
the requirement of something we can produce in payment so as to obtain the thing we need.
Money, it is true, is liable to the same fluctuation of demand as other commodities, for
its purchasing power varies at different times; but it tends to be comparatively constant.
Hence the proper thing is for all commodities to have their prices fixed; this will ensure
that exchange, and consequently association, shall always be possible. Money then serves
as a measure which makes things commensurable and so reduces them to equality. If there
were no exchange there would be no association, and there can be no exchange without
equality, and no equality without commensurability. Though therefore it is impossible for
things so different to become commensurable in the strict sense, our demand furnishes a sufficiently accurate common measure for
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.