[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 § 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 mediæval 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 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