The Complete Aristotle (eng.)

‘reciprocity’ fits neither
distributive nor rectificatory justice-yet people want even the
justice of Rhadamanthus to mean this:
    Should a man suffer what he did, right justice would be done
-for in many cases reciprocity and rectificatory justice are not in
accord; e.g. (1) if an official has inflicted a wound, he should
not be wounded in return, and if some one has wounded an official,
he ought not to be wounded only but punished in addition. Further
(2) there is a great difference between a voluntary and an
involuntary act. But in associations for exchange this sort of
justice does hold men together-reciprocity in accordance with a
proportion and not on the basis of precisely equal return. For it
is by proportionate requital that the city holds together. Men seek
to return either evil for evil-and if they cana not do so, think
their position mere slavery-or good for good-and if they cannot do
so there is no exchange, but it is by exchange that they hold
together. This is why they give a prominent place to the temple of
the Graces-to promote the requital of services; for this is
characteristic of grace-we should serve in return one who has shown
grace to us, and should another time take the initiative in showing
it.
    Now proportionate return is secured by cross-conjunction. Let A
be a builder, B a shoemaker, C a house, D a shoe. The builder,
then, must get from the shoemaker the latter’s work, and must
himself give him in return his own. If, then, first there is
proportionate equality of goods, and then reciprocal action takes
place, the result we mention will be effected. If not, the bargain
is not equal, and does not hold; for there is nothing to prevent
the work of the one being better than that of the other; they must
therefore be equated. (And this is true of the other arts also; for
they would have been destroyed if what the patient suffered had not
been just what the agent did, and of the same amount and kind.) For
it is not two doctors that associate for exchange, but a doctor and
a farmer, or in general people who are different and unequal; but
these must be equated. This is why all things that are exchanged
must be somehow comparable. It is for this end that money has been
introduced, and it becomes in a sense an intermediate; for it
measures all things, and therefore the excess and the defect-how
many shoes are equal to a house or to a given amount of food. The
number of shoes exchanged for a house (or for a given amount of
food) must therefore correspond to the ratio of builder to
shoemaker. For if this be not so, there will be no exchange and no
intercourse. And this proportion will not be effected unless the
goods are somehow equal. All goods must therefore be measured by
some one thing, as we said before. Now this unit is in truth
demand, which holds all things together (for if men did not need
one another’s goods at all, or did not need them equally, there
would be either no exchange or not the same exchange); but money
has become by convention a sort of representative of demand; and
this is why it has the name ‘money’ (nomisma)-because it exists not
by nature but by law (nomos) and it is in our power to change it
and make it useless. There will, then, be reciprocity when the
terms have been equated so that as farmer is to shoemaker, the
amount of the shoemaker’s work is to that of the farmer’s work for
which it exchanges. But we must not bring them into a figure of
proportion when they have already exchanged (otherwise one extreme
will have both excesses), but when they still have their own goods.
Thus they are equals and associates just because this equality can
be effected in their case. Let A be a farmer, C food, B a
shoemaker, D his product equated to C. If it had not been possible
for reciprocity to be thus effected, there would have been no
association of the parties. That demand holds things together as a
single unit is shown by the fact that when men do not need one
another, i.e. when neither needs the other or one does not need the
other, they do not exchange, as we do when some one wants what one
has oneself, e.g. when people permit the exportation of corn in
exchange for wine. This equation therefore must be established. And
for the future exchange-that if we do not need a thing now we shall
have it if ever we do need it-money is as it were our surety; for
it must be possible for us to get what we want by bringing the
money. Now the same thing happens to money itself as