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Usury is a sin against the natural law and punishable as heresy
"He that suffers injury does not sin, according to the Philosopher (Ethic. v, 11), wherefore justice is not a mean between two vices, as stated in the same book (ch. 5). Now a usurer sins by doing an injury to the person who borrows from him under a condition of usury. Therefore he that accepts a loan under a condition of usury does not sin."
But is this not inducing a man to sin?
"It is by no means lawful to induce a man to sin, yet it is lawful to make use of another's sin for a good end, since even God uses all sin for some good, since He draws some good from every evil as stated in the Enchiridion (xi). Hence when Publicola asked whether it were lawful to make use of an oath taken by a man swearing by false gods (which is a manifest sin, for he gives Divine honor to them) Augustine (Ep. xlvii) answered that he who uses, not for a bad but for a good purpose, the oath of a man that swears by false gods, is a party, not to his sin of swearing by demons, but to his good compact whereby he kept his word. If however he were to induce him to swear by false gods, he would sin."
Accordingly we must also answer to the question in point that it is by no means lawful to induce a man to lend under a condition of usury: yet it is lawful to borrow for usury from a man who is ready to do so and is a usurer by profession; provided the borrower have a good end in view, such as the relief of his own or another's need. Thus too it is lawful for a man who has fallen among thieves to point out his property to them (which they sin in taking) in order to save his life, after the example of the ten men who said to Ismahel (Jeremiah 41:8): "Kill us not: for we have stores in the field."
But isn't it unlawful to consent to the sin of usury, or any other sin?
"He who borrows for usury does not consent to the usurer's sin but makes use of it. Nor is it the usurer's acceptance of usury that pleases him, but his lending, which is good."
But does he not give the usurer an occasion of sin?
"He who borrows for usury gives the usurer an occasion, not for taking usury, but for lending; it is the usurer who finds an occasion of sin in the malice of his heart."
Is it unlawful to deposit one's money with a usurer?
"If one were to entrust one's money to a usurer lacking other means of practising usury; or with the intention of making a greater profit from his money by reason of the usury, one would be giving a sinner matter for sin, so that one would be a participator in his guilt. If, on the other hand, the usurer to whom one entrusts one's money has other means of practising usury, there is no sin in entrusting it to him that it may be in safer keeping, since this is to use a sinner for a good purpose."