The Wealth of Nations by Part 2 Chapter 4 Page 10

revenue, without being at the trouble of employing them themselves, naturally accompanies the general increase of capitals; or, in other words, as stock increases, the quantity of stock to be lent at interest grows gradually greater and greater.

As the quantity of stock to be lent at interest increases, the interest, or the price which must be paid for the use of that stock, necessarily diminishes, not only from those general causes which make the market price of things commonly diminish as their quantity increases, but from other causes which are peculiar to this particular case. As capitals increase in any country, the profits which can be made by employing them necessarily diminish. It becomes gradually more and more difficult to find within the country a profitable method of employing any new capital.