were those that applied to sale. Indeed
all sales when analysed are really barter through the medium of
money. That Aquinas simply regarded his article on just price[1] as an
explanation of the application of his general teaching on justice to
the particular case of the contract of sale is quite clear from the
article itself. 'Apart from fraud, we may speak of buying and selling
in two ways. First, as considered in themselves; and from this point
of view buying and selling seem to be established for the common
advantage of both parties, one of whom requires that which belongs
to the other, and _vice versa_. Now whatever is established for the
common advantage should not be more of a burden to one part than to
the other, and consequently all contracts between them should observe
equality of thing and thing. Again, the quality of a thing that
comes into human use is measured by the price given for it, for which
purpose money was invented. Therefore, if either the price exceed the
quantity of the thing's worth, or conversely the worth of the thing
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