Capital, Volume Three: The Process of Capitalist Production as a Whole by Karl Marx

Interest, as we have seen in the two preceding chapters, appears originally, is originally, and remains in fact merely a portion of the profit, i.e., of the surplus-value, which the functioning capitalist, industrialist or merchant has to pay to the owner and lender of money-capital whenever he uses loaned capital instead of his own. If he employs only his own capital, no such division of profit takes place; the latter is then entirely his. Indeed, as long as the owners of the capital employ it on their own in the reproduction process, they do not compete in determining the rate of interest. This alone shows that the category of interest – impossible without determining the rate of interest – is alien to the movements of industrial capital as such.

"The rate of interest may be defined to be that proportional sum which the lender is content to receive, and the borrower to pay, annually, or for any longer or shorter period, for the use of a certain amount of moneyed capital. When the owner of a capital employs it actively in reproduction, he does not come under the head of those capitalists, the proportion of whom, to the number of borrowers, determines the rate of interest."; (Th. Tooke, History of Prices, London, 1838, II, pp. 355-56.)