Letter 81 (§18)

“For ungrateful men make mistakes in this respect also: they have to pay their creditors both capital and interest, but they think that benefits are currency which they can use without interest. So the debts grow through postponement, and the later the action is postponed the more remains to be paid. A man is an ingrate if he repays a favour without interest. Therefore, interest also should be allowed for, when you compare your receipts and your expenses.”