would seem chiefly to be opposed to justice which consists in
equality. Now quantity is known by being measured: and the measures
of things that come into human use are not fixed, but in some places
are greater, in others less, as the Philosopher states (Ethic. v, 7).
Therefore just as it is impossible to avoid defects on the part of
the thing sold, it seems that a sale is not rendered unlawful through
the thing sold being defective.

Obj. 3: Further, the thing sold is rendered defective by lacking a
fitting quality. But in order to know the quality of a thing, much
knowledge is required that is lacking in most buyers. Therefore a
sale is not rendered unlawful by a fault (in the thing sold).

_On the contrary,_ Ambrose says (De Offic. iii, 11): "It is
manifestly a rule of justice that a good man should not depart from
the truth, nor inflict an unjust injury on anyone, nor have any
connection with fraud."

_I answer that,_ A threefold fault may be found pertaining to the
thing which is sold. One, in respect of the thing's substance: and if
the seller be aware of a fault in the thing he is selling, he is
guilty of a fraudulent sale, so that the sale is rendered unlawful.
Hence we find it written against certain people (Isa. 1:22), "Thy
silver is turned into dross, thy wine is mingled with water": because
that which is mixed is defective in its substance.

Another defect is in respect of quantity which is known by being
measured: wherefore if anyone knowingly make use of a faulty measure
in selling, he is guilty of fraud, and the sale is illicit. Hence it
is written (Deut. 25:13, 14): "Thou shalt not have divers weights in
thy bag, a greater and a less: neither shall there be in thy house a
greater bushel and a less," and further on (Deut. 25:16): "For the
Lord . . . abhorreth him that doth these things, and He hateth all
injustice."

A third defect is on the part of the quality, for instance, if a man
sell an unhealthy animal as being a healthy one: and if anyone do
this knowingly he is guilty of a fraudulent sale, and the sale, in
consequence, is illicit.

In all these cases not only is the man guilty of a fraudulent sale,
but he is also bound to restitution. But if any of the foregoing
defects be in the thing sold, and he knows nothing about this, the
seller does not sin, because he does that which is unjust materially,
nor is his deed unjust, as shown above (Q. 59, A. 2). Nevertheless he
is bound to compensate