I respond, it is to be said that to give money in loan at usury is a mortal sin. Nor is it only a sin because it is prohibited; but rather is prohibited because it is secundum se a sin. For it is against natural justice. And this is manifest, if we should rightly consider the account of usury. For it is called usury from use, fom this, to wit, that a certain price is accepted for the use of money, as if the use itself of the money lent were being sold. It is to be considered, however, that of diverse things there is a diverse use. For there are certain things of which the use is the consumption of the substance of the things themselves, as the proper use of wine is that it should be drunk, and in this the substance of the wine is consumed; and likewise the proper use of wheat or bread is that is should be eaten, which is the consumption of the wheat itself or bread; so also the proper use of money is that it should be expended for the exchange of other things. For numismata are invented for the sake of exchange, as the philosopher says in Politics, VII. But there are certain things the use of which is not the consumption of the substance of the thing itself, as the use of a house is inhabitation. It is not, however, of the account of inhabitation that the house should be destroyed; if however it should happen that the house by the inhabiting should be in something bettered or worsened, this is per accidens; and the same is to be said about a horse or a cloak, and other things of this sort. Because, however, a thing of this sort is not consumed through use strictly speaking, therefore differently either the thing itself or the use is able to be given or sold, or either together. For someone is able to sell a house, retaining to himself the use of the house for a time; and likewise someone is able to sell the use of a house, retaining to himself the property and ownership of the house. But in those things of which the use is consumption, the use of the thing is not other than the thing itself, whence to whomever is given the use of such a thing, is given also the ownership of the thing itself, and conversely.
When, therefore, someone lends money under this pact, that the money should be restored to him whole, and further wishes to have a certain price for the use of the money, it is manifest that he sells separately the use of the money and the substance itself of the money. The use, however, of money, as has been said, is not other than its substance; whence either he sells what is not, or else he sells the same twice, to wit the money itself of which the use is consumption of it. And this is manifestly against the account of natural justice. Whence, to lend money for usury is secundum se a mortal sin: and there is the same account regarding all other things of which the substance is consumed through use; as is manifest in wine, wheat, and other things of this sort.