Read "Capital, Volume Three: The Process of Capitalist Production as a Whole" by Karl Marx online for free on Textopian. Full text with search, annotations, highlights, and AI-powered reading aids.
Interest-bearing capital, or, as we may call it in its antiquated form, usurer's capital, belongs together with its twin brother, merchant's capital, to the antediluvian forms of capital, which long precede the capitalist mode of production and are to be found in the most diverse economic formations of society.
The existence of usurer's capital merely requires that at least a portion of products should be transformed into commodities, and that money should have developed in its various functions along with trade in commodities.