Aquinas on usury: https://www.newadvent.org/summa/3078.htm
> On the contrary, It is written (Exodus 22:25): "If thou lend money to any of thy people that is poor, that dwelleth with thee, thou shalt not be hard upon them as an extortioner, nor oppress them with usuries."
> I answer that, To take usury for money lent is unjust in itself, because this is to sell what does not exist, and this evidently leads to inequality which is contrary to justice. In order to make this evident, we must observe that there are certain things the use of which consists in their consumption: thus we consume wine when we use it for drink and we consume wheat when we use it for food. Wherefore in such like things the use of the thing must not be reckoned apart from the thing itself, and whoever is granted the use of the thing, is granted the thing itself and for this reason, to lend things of this kin is to transfer the ownership. Accordingly if a man wanted to sell wine separately from the use of the wine, he would be selling the same thing twice, or he would be selling what does not exist, wherefore he would evidently commit a sin of injustice. On like manner he commits an injustice who lends wine or wheat, and asks for double payment, viz. one, the return of the thing in equal measure, the other, the price of the use, which is called usury.
Since it is "selling what does not exist", this forces the production of new items to fill this need. It's basically the "creation" of debt. If a person is loaned a car, they can give the tangible car back, it is an actual thing. If a person is loaned a car with interest, the interest does not actually exist, it is a fiction (until someone uses labor to creat something to then make the thing a reality). Likewise, "interest" on investments is different, because it is an actual thing. Like if you plant a fruit tree, kind of like owning a "natural stock", and it produces a "dividend" of fruit the next year, those are actual tangible goods / profits. Unlike, say, loaning a tree and then charging a certain percent of interest on the loan based on predictions of what fruit the tree may or may not produce (it may not produce anything, thus making the fictional interest "unpayable"). This kind of thing leads to distortions as actual production may be out of sync with fictional interest rates.
To me this is the heart of the problem with interest on loans, although I think I've seen other people argue there's some other problem which makes interest-bearing loans wrong.
The Biblical ideal is to loan without expecting anything back, like giving a coat away and not even expecting it to be returned, although in justice it could be asked that it be returned if necessary: "But love ye your enemies: do good, and lend, hoping for nothing thereby" Luke 6:35
> On the contrary, It is written (Exodus 22:25): "If thou lend money to any of thy people that is poor, that dwelleth with thee, thou shalt not be hard upon them as an extortioner, nor oppress them with usuries."
> I answer that, To take usury for money lent is unjust in itself, because this is to sell what does not exist, and this evidently leads to inequality which is contrary to justice. In order to make this evident, we must observe that there are certain things the use of which consists in their consumption: thus we consume wine when we use it for drink and we consume wheat when we use it for food. Wherefore in such like things the use of the thing must not be reckoned apart from the thing itself, and whoever is granted the use of the thing, is granted the thing itself and for this reason, to lend things of this kin is to transfer the ownership. Accordingly if a man wanted to sell wine separately from the use of the wine, he would be selling the same thing twice, or he would be selling what does not exist, wherefore he would evidently commit a sin of injustice. On like manner he commits an injustice who lends wine or wheat, and asks for double payment, viz. one, the return of the thing in equal measure, the other, the price of the use, which is called usury.
Since it is "selling what does not exist", this forces the production of new items to fill this need. It's basically the "creation" of debt. If a person is loaned a car, they can give the tangible car back, it is an actual thing. If a person is loaned a car with interest, the interest does not actually exist, it is a fiction (until someone uses labor to creat something to then make the thing a reality). Likewise, "interest" on investments is different, because it is an actual thing. Like if you plant a fruit tree, kind of like owning a "natural stock", and it produces a "dividend" of fruit the next year, those are actual tangible goods / profits. Unlike, say, loaning a tree and then charging a certain percent of interest on the loan based on predictions of what fruit the tree may or may not produce (it may not produce anything, thus making the fictional interest "unpayable"). This kind of thing leads to distortions as actual production may be out of sync with fictional interest rates.
To me this is the heart of the problem with interest on loans, although I think I've seen other people argue there's some other problem which makes interest-bearing loans wrong.
The Biblical ideal is to loan without expecting anything back, like giving a coat away and not even expecting it to be returned, although in justice it could be asked that it be returned if necessary: "But love ye your enemies: do good, and lend, hoping for nothing thereby" Luke 6:35