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 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 words, they assume the existence of property of external goods in individuals.  Commutations are but a result of private property; in a state of communism there could be no commutation.  This is well pointed out by Gerson[1] and by Nider.[2] It consequently is important, before discussing exchange of ownership, to discuss the principle of ownership itself; or, in other words, to study the static before the dynamic state.[3]

[Footnote 1:  De Contractibus, i. 4 ’Inventa est autem commutatio civilis post peccatum quoniam status innocentias habuit omnia communia.’]

[Footnote 2:  De Contractibus, v. 1:  ’Nunc videndum est breviter unde originaliter proveniat quod rerum dominia sunt distincta, sic quod hoc dicatur meum et illud tuum; quia illud est fundamentum omnis injustitiae in contractando rem alienam, et post omnis injustitia reddendo eam.’]

[Footnote 3:  See l’Abbe Desbuquois, op. cit., p. 168.]