The scholastic teaching on the subject of the rules of justice in
exchange was founded on the famous fifth book of Aristotle's _Ethics_,
and is very clearly set forth by Aquinas. In the article of the
_Summa_, where the question is discussed, 'Whether the mean is to be
observed in the same way in distributive as in commutative justice?'
we find a clear exposition: 'In commutations something is delivered to
an individual on account of something of his that has been received,
as may be seen chiefly in selling and buying, where the notion of
commutation is found primarily. Hence it is necessary to equalise
thing with thing, so that the one person should pay back to the other
just so much as he has become richer out of that which belonged to
the other. The result of this will be equality according to the
_arithmetical_ mean, which is gauged according to equal excess in
quantity. Thus 5 is the mean between 6 and 4, since it exceeds the
latter, and is exceeded by the former by 1. Accordingly, if at the
start both persons have 5, and one of them receives 1 out of the
other's belongings, the one that is the receiver will have 6, and the
other will be left with 4: and so there will be justice if both are
brought back to the mean, I being taken from him that has 6 and given
to him that has 4, for then both will have 5, which is the mean.'[1]
In the following article the matter of each kind of justice is
discussed. We are told that: 'Justice is about certain external
operations, namely, distribution and commutation. These consist in the
use of certain externals, whether things, persons, or even works: of
things as when one man takes from or restores to another that which
is his: of persons as when a man does an injury to the very person of
another...: and of works as when a man justly enacts a work of another
or does a work for him.... Commutative justice directs commutations
that can take place between two persons. Of these some are
involuntary, some voluntary.... Voluntary commutations are when a