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.]

We shall therefore deal in the first place with the right of private
property, which we shall show to have been fully recognised by the
mediaeval writers. We shall then point out the duties which this
right entailed, and shall establish the position that the scholastic
teaching was directed equally against modern socialistic principles
and modern unregulated individualism. The next point with which we
shall deal is the exchange of property between individuals, which is a
necessary corollary of the right of property. We shall show that such
exchanges were regulated by well-defined principles of commutative
justice, which applied equally in the case of the sale of goods and in
the case of the sale of the use of money. The last matter with which
we shall deal is the machinery by which exchanges are conducted,
namely, money. Many other subjects, such as slavery and the legitimacy
of commerce, will be treated as they arise in the course of our
treatment of these principal divisions.

In its ultimate analysis, the whole subject may be reduced to a
classification of the various duties which attached to the right of
private propert