The Wealth of Nations by Part 1 Chapter 9 Page 28

The proportion which the usual market rate of interest ought to bear to the ordinary rate of clear profit, necessarily varies as profit rises or falls. Double interest is in Great Britain reckoned what the merchants call a good, moderate, reasonable profit; terms which I apprehend mean no more than a common and usual profit. In a country where the ordinary rate of clear profit is eight or ten per cent, it may be reasonable that one half of it should go to interest, wherever business is carried on with borrowed money.

The stock is at the risk of the borrower, who, as it were, insures it to the lender; and four or five per cent may, in the greater part of trades, be both a sufficient profit upon the risk of this insurance, and a sufficient recompense for the trouble of employing the stock. But the proportion between