Book 5.21
"But," argues he, "by not authorizing you, the law forbids you to exact repayment." There are many things which are not enforced by any law or process, but which the conventions of society, which are stronger than any law, compel us to observe. There is no law forbidding us to divulge our friend's secrets; there is no law which bids us keep faith even with an enemy; pray what law is there which binds us to stand by what we have promised? There is none. Nevertheless I should remonstrate with one who did not keep a secret, and I should be indignant with one who pledged his word and broke it. "But," he argues, "you are turning a benefit into a loan." By no means, for I do not insist upon repayment, but only demand it; nay, I do not even demand it, but remind my friend of it. Even the direst need will not bring me to apply for help to one with whom I should have to undergo a long struggle.
If there be any one so ungrateful that it is not sufficient to remind him of his debt, I should pass him over, and think that he did not deserve to be made grateful by force. A money-lender does not demand repayment from his debtors if he knows they have become bankrupt, and, to their shame, have nothing but shame left to lose; and I, like him, should pass over those who are openly and obstinately ungrateful, and would demand repayment only from those who were likely to give it me, not from those from whom I should have to extort it by force.