The fundamental insight of Aquinas with regard to usury is that it involves double charging or charging for what does not exist. Whatever defense of charging interest for a money loan one might give, this is the fundamental insight. This manifestly unjust whatever one believes about money. Even if you wish to claim that in some relevant sense money is capital, it is fundamental to the nature of the money that it is consumed in its use, whether it is used to easily purchase capital or not. To charge interest on the money loan itself is unjust in virtue of the fact that the money cannot be separated from its use. It is a double charging to charge interest on such a loan. Moreover, it is charging for what does not exist to charge for the potential (i.e. not actual) capital that might or might not be exchanged for the money. This then is the fundamental problem for the usurer and it is not waived away by suggesting that circumstance have changed.
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