hich 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
man voluntarily transfers his chattel to another person. And if he
transfer it simply so that the recipient incurs no debt, as in the
case of gifts, it is an act not of justice, but of liberality. A
voluntary transfer belongs to justice in so far as it includes the
notion of debt.' Aquinas then goes on to distinguish between the
different kinds of contract, sale, usufruct, loan, letting and hiring,
and deposit, and concludes, 'In all these actions the mean is taken in
the same way according to the equality of repayment. Hence all these
actions belong to the one species of justice, namely, commutative
justice.'[2]

[Footnote 1: ii. ii. 61, 2.]

[Footnote 2: ii. ii. 61, 3. The reasoning of Aristotle is
characteristically reinforced by the quotation of Matt. vii. 12; ii.
ii. 77,1.]

This is not the place to discuss the precise meaning of the equality
upon which Aquinas insists, which will be more properly considered
when we come to deal with the just price. What is to be noticed at
present is that all the transactions which are properly comprised in
a discussion of economic theory--sales, loans, etc.--are grouped
together as being subject to the same