Usury and Capitalism (short version)

"Usury", in its simplest meaning, means "to charge for the use of a thing."  But Aquinas points out that the ACTUAL definition of usury is "to sell that which does not exist", i.e., to commit what we would now call "fraud."

In a subsistence-level society, money is functionally identical to consumption. In a surplus-goods society, money is NOT functionally identical to consumption.

Thus, as the monasteries were able to produce local areas of surplus goods, the function of money changed and the definition changed with it.

See Aquinas' commentary here as he explains why you can charge for the use of some things (e.g., you can rent a house), but not for the use of other things (you can't rent food).

Money was originally understood to be very much like food, but over time, the medievals began to realize that surplus food made it possible to think of money more like a house. It was this change in the understanding and use of money which allows one to "build a house of gold", i.e., accumulate capital.

I have a longer gloss here on his commentary which may (or may not) help to understand what he meant.

So, both capitalism and the industrial revolution are the result of Christian philosophy. The Catholic willingness to understand that the definition of money could change, and the affect this change had on the definition of usury was central to the development of capitalism. The Catholic development of experimental science was built off the high regard Catholics have always had for manual labor, and central to the industrial revolution.

Islam never understood the change in the definition of  "money" from the 600s through the 1500 years that followed. Christians, on the other hand, realized that the definition being used in their Scriptures was fundamentally different than how the word was being used even by the end of the first millennium. Thus, you see Aquinas drawing very fine distinctions in the Summa Theologica about what does and does not constitute usury.

Precisely because the definition of "money" changed, the definition of what it meant to "charge interest" also changed, and Aquinas recognized those changes. Usury is still a sin to this day, but Christians don't use the 600s AD broad-based definition still used today by Muslim banks, which inhibits capital formation in Muslim countries.

Instead, Christians recognized the elements of the underlying economy, distinguished them, and pointed out that many uses of this new "money" paradigm didn't actually violate any spiritual principles that had been put in peril under the old definitions.

The change then was almost as profound as the move from gold-based to fiat-based to crypto. Christians recognized, tracked and leveraged these changes without violating the spirit of the Faith. Muslims still haven't figured out that anything is different.