The capacity of an object for satisfying man's needs could not be measured by its capacity for satisfying the needs of this or that individual, but by its capacity for satisfying the needs of the average member of the community.[1] The Abbé Desbuquois, in the article from which we have already quoted, finds in this elevation of the common estimation an illustration of the general principle of the mediævals, which we have seen at work in their teaching on the use of property, that the individual benefit must always be subordinated to the general welfare. According to him, it is but one application of the duty of using one's goods for the common good. 'In the same way, in allowing the right of exchange--a right, let us remark in passing, which is but an application of the right of property--and in allowing it as a means of life necessary to everybody, nature does not lose sight of the universal destination of economic goods. One conceives then that the variations of exchange are not permitted to be left to the arbitrary judgment of a single man, nor to be affected by the whims and abuses of individuals; that value is defined in view of the general good. The exchange value, as it is in the general or social order, proceeds from the judgment of the social environment (_milieu social_).'[2] [Footnote 1: 'Indigentia istius hominis vel illius non mensurat valorem commutabilium; sed indigentia communis eorum qui inter se commutare possunt,' Buridan, _op. cit._, v. 16. 'Prout communiter venditur in foro,' Henri de Gand, _Quod Lib._, xiv. 14; Nider, _De Cont. Merc._, ii. 1.] [Footnote 2: 'La Justice dans l'Echange,' _Semaine Sociale de France_, 1911, p. 168.] The writers of the Middle Ages show a very keen perception of the elements which invest an object with the value which is accorded to it by the general estimation. In Aquinas we find certain elements recognised--'diversitas loci vel temporis, labor, raritas'--but it is not until the authors of the fourteenth and fifteenth centuries that we find a systematic treatment of value.[1] First and foremost there is the cost of production of the article, especially the wages of all those who helped to produce it. Langenstein lays down that every one can determine for himself the just price of the wares he has to sell by reckoning what he needs to support himself in the status which he occupies.[2] According to the _Catholic Encyclopædia_,[3] the just price of an article included enough to pay fair wages to the worker--that is, enough to enable him to maintain the standard of living of his class. This, though not stated in so many words by Aquinas, was probably assumed by him as too obvious to need repetition.[4] 'The cost of production of manufactured products,' says Brants, 'is a legitimate constituent element of value; it is according to the cost that the producer can properly fix the value of his product and of his work.'[5] [Footnote 1: Brants, _op. cit._, p. 69.]