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'Abbé 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 mediæval 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.