Thomas Aquinas

“ If a man were, in return for money lent, as though there had been an agreement tacit or expressed, to expect or exact repayment in the shape of some remuneration of service or words, it would be the same as if he expected or exacted some real remuneration, because both can be priced at a money value, as may be seen in the case of those who offer for hire the labor which they exercise by work or by tongue. ”

“ Human laws leave certain things unpunished, on account of the condition of those who are imperfect, and who would be deprived of many advantages, if all sins were strictly forbidden and punishments appointed for them. Wherefore human law has permitted usury, not that it looks upon usury as harmonizing with justice, but lest the advantage of many should be hindered. ”

“ Money cannot be sold for a greater sum than the amount lent, which has to be paid back: nor should the loan be made with a demand or expectation of aught else but of a feeling of benevolence which cannot be priced at a pecuniary value, and which can be the basis of a spontaneous loan. ”

“ Now to lend money to one who is in straits is to do him a favor for which he should be grateful. Therefore the recipient of a loan, is bound by a natural debt to repay something. Now it does not seem unlawful to bind oneself to an obligation of the natural law. Therefore it is not unlawful, in lending money to anyone, to demand some sort of compensation as condition of the loan.
Objection 3: Further, just as there is real remuneration, so is there verbal remuneration, and remuneration by service, as a gloss says on Is. ”

“ Hence there is passive scandal on his part, while there is no active scandal on the part of the person who seeks to borrow. Nor is this passive scandal a reason why the other person should desist from borrowing if he is in need, since this passive scandal arises not from weakness or ignorance but from malice. ”

“ It must be observed, however, that the secondary use of silver vessels may be an exchange, and such use may not be lawfully sold. In like manner there may be some secondary use of silver money; for instance, a man might lend coins for show, or to be used as security. ”