the buyer, when the defect comes to his
knowledge. Moreover what has been said of the seller applies equally
to the buyer. For sometimes it happens that the seller thinks his
goods to be specifically of lower value, as when a man sells gold
instead of copper, and then if the buyer be aware of this, he buys it
unjustly and is bound to restitution: and the same applies to a
defect in quantity as to a defect in quality.

Reply Obj. 1: Gold and silver are costly not only on account of the
usefulness of the vessels and other like things made from them, but
also on account of the excellence and purity of their substance.
Hence if the gold or silver produced by alchemists has not the true
specific nature of gold and silver, the sale thereof is fraudulent
and unjust, especially as real gold and silver can produce certain
results by their natural action, which the counterfeit gold and
silver of alchemists cannot produce. Thus the true metal has the
property of making people joyful, and is helpful medicinally against
certain maladies. Moreover real gold can be employed more frequently,
and lasts longer in its condition of purity than counterfeit gold. If
however real gold were to be produced by alchemy, it would not be
unlawful to sell it for the genuine article, for nothing prevents art
from employing certain natural causes for the production of natural
and true effects, as Augustine says (De Trin. iii, 8) of things
produced by the art of the demons.

Reply Obj. 2: The measures of salable commodities must needs be
different in different places, on account of the difference of
supply: because where there is greater abundance, the measures are
wont to be larger. However in each place those who govern the state
must determine the just measures of things salable, with due
consideration for the conditions of place and time. Hence it is not
lawful to disregard such measures as are established by public
authority or custom.

Reply Obj. 3: As Augustine says (De Civ. Dei xi, 16) the price of
things salable does not depend on their degree of nature, since at
times a horse fetches a higher price than a slave; but it depends on
their usefulness to man. Hence it is not necessary for the seller or
buyer to be cognizant of the hidden qualities of the thing sold, but
only of such as render the thing adapted to man's use, for instance,
that the horse be strong, run well and so forth. Such qualities the
seller and buyer can easily discover.