Biel, _op. cit._, iv. xv. 5.]

[Footnote 5: _Op. cit._, p. 83.]

On the other hand, it must not be imagined, as has sometimes been
suggested, that the slavery defended by Aquinas was not real slavery,
but rather the ordinary modern relation between employer and employed.
Such an interpretation is definitely disproved by a passage of the
article on justice where Aquinas says that 'inducing a slave to leave
his master is properly an injury against the person ... and, since the
slave is his master's chattel, it is referred to theft.'[1]

[Footnote 1: II. ii. 61,3. Brants, _op. cit._, pp. 87 _et seq_., is
inclined to take a more liberal view of the scholastic doctrine on
slavery, but we cannot agree with him in view of the contemporary
texts.]

CHAPTER III

DUTIES REGARDING THE EXCHANGE OF PROPERTY

SECTION 1.--THE SALE OF GOODS

Sec. 1. _The Just Price_.

We dealt in the last chapter with the duties which attached to
property in respect of its acquisition and use, and we now pass to
the duties which attached to it in respect of its exchange. As we
indicated above, the right to exchange one's goods for the goods or
the money of another person was, according to the scholastics, one of
the necessary corollaries of the right of private property. In order
that such exchange might be justifiable, it must be conducted on a.
basis of commutative justice, which, as we have seen, consisted in the
observance of equality according to the arithmetical mean. We further
drew attention to the fact that exchanges might be divided into
sales of goods and sales of the use of money. In the former case the
regulating principle of the equality of justice was given effect to
by the observance of the _just price_; in the latter by that of the
_prohibition of usury_. We shall deal with the former in the present
and with the latter in the following section.

The mediaeval teaching on the just price, about which there has been so
much discussion and disagreement among modern writers, was simply the
application to the particular contract of sale of the principles which
regulated contracts in general. Exchange originally took the form
of barter; but, as it was found impossible accurately to measure
the values of the objects exchanged without the intervention of
some common measure of value, money was invented to serve as such a
measure. We need not further refer to barter in this section, as the
principles which applied to it