Interest, signifying the price of capital, is from the outset quite an irrational expression. The commodity in question has a double value, first a value, and then a price different from this value, while price represents the expression of value in money.
 Karl Marx, Das Kapital (1894). copy citation

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Author Karl Marx
Source Das Kapital
Topic money value
Date 1894
Language English
Reference
Note
Weblink https://www.marxists.org/archive/marx/works/download/pdf/Capital-Volume-...

Context

“[6] The price is here reduced to its purely abstract and meaningless form, signifying that it is a certain sum of money paid for something serving in one way or another as a use-value; whereas the conception of price really signifies the value of some use-value expressed in money.
Interest, signifying the price of capital, is from the outset quite an irrational expression. The commodity in question has a double value, first a value, and then a price different from this value, while price represents the expression of value in money. Money-capital is nothing but a sum of money, or the value of a certain quantity of commodities fixed in a sum of money. If a commodity is loaned out as capital, it is only a disguised form of a sum of money.” source