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 regulative principle. It
therefore appears more correct to approach the subject which we are
attempting to treat by following that principle into its various
applications, than by making one particular application of the
principle the starting-point of the discussion.

It will be noticed, however, that the principles of commutative
justice all treat of the commutations of external goods--in other