Thomas Aquinas discusses usury in a very clear argument in the Summa Theologiae (IIª-IIae q. 78 a. 1). I summarise his argument as follows. He says that it is unjust to sell the same thing twice, but usury is selling the same thing twice, therefore usury is unjust. The minor premiss is proved as follows. Usury is selling the use of money, and demanding the same money back. Now the use of money consists in its consumption - according to Aristotle (Ethic. v, 5; Polit. i, 3), money was invented chiefly for the purpose of exchange: "and consequently the proper and principal use of money is its consumption or alienation whereby it is sunk in exchange". But to sell the use of a thing whose use consists in its consumption, and to demand the same thing back, is selling the same thing twice, therefore etc.
Thomas compares usury to selling wine separately from the use of the wine. This is to be distinguished from renting a house, where the use consists in dwelling it, not in destroying it. To rent out a house is not selling the same thing twice, and therefore not unjust.
The argument seems clear and forceful. But it seems to contradict the conclusion I came to in a previous post, namely that money does have an intrinsic 'time value'. What has gone wrong? More tomorrow.