exceed the price, there is no longer the equality of justice; and consequently to sell a thing for more than its worth, or to buy it for less than its worth, is in itself unjust and unlawful.'[2] When two contracting parties make an exchange through the medium of money, the price is the expression of the exchange value in money. 'The just price expresses the equivalence, which is the foundation of contractual justice.'[3] [Footnote 1: II. ii. 77, 1.] [Footnote 2: This opinion was accepted by all the later writers, _e.g._ Gerson, _De Cont._, ii. 5; Biel, _op. cit._, IV. xv. 10: 'Si pretium excedit quantitatem valoris rei, vel e converso tolleretur equalitas, erit contractus iniquus.'] [Footnote 3: Desbuquois, 'La Justice dans l'Echange,' _Semaine Sociale de France_, 1911, p. 167. Gerson says: 'Contractus species est justitiae commutativae quae respicit aequalitatem rei quae venditur ad rem quae emitur, ut servetur aequalitas justi pretii; propter quam aequalitatem facilius observandum inventa est moneta, vel numisma, vel pecunia,' _De Cont._, ii. 5.] The conception of the just price, though based on Aristotelian conceptions of justice, is essentially Christian. The Roman law had allowed the utmost freedom of contract in sales; apart from fraud, the two contracting parties were at complete liberty to fix a price at their own risk; and selfishness was assumed and allowed to be the animating motive of every contracting party. The one limitation to this sweeping rule was in favour of the seller. By a rescript of Diocletian and Maximian it was enacted that, if a thing were sold for less than half its value, the seller could recover the property, unless the buyer chose to make up the price to the full amount. Although this rescript was perfectly general in its terms, some authors contended that it applied only to sales of land, because the example given was the sale of a farm.[1] However, the rescript was quoted by the Fathers as showing that even the Roman law considered that contracts might be questioned on equitable grounds in certain cases.[2] The distinctively Christian notion of just price seems to have its origin in a passage of St. Augustine;[3] but the notion was not placed on a philosophical foundation until the thirteenth century. Even Aquinas, however, although he treats of the just price at some length, and expresses clear and categorical opinions upon many points connected with it, does not state the principles on which the just price itself should be arrived at. This omission is due, not to the fact that Aquinas was unfamiliar with these principles, but to the fact that he took them for granted as they were not disputed or doubted.[4] We have consequently to look for enlightenment upon this point in writings other than those of Aquinas. The subject can be most satisfactorily understood if we divide its treatment into two parts: first, a consideration of what constituted the just price in the sale