Book II, Chapter VIII: Of the Revenue of Capital.

The service, rendered by capital, in productive operations, establishes a demand for capital to be so employed, and enables the proprietors of it to charge more or less for that service.

Whether the capitalist thus employ his capital himself, or lend it to another for that purpose, it yields a profit, that is called the profit of capital, distinct from that of the industry employing it. In the former case, the profit obtained constitutes the revenue of his capital, which is added to that of his personal talent and industry, and often confounded with it.—In the latter, the revenue of capital is precisely the interest paid for its use, the proprietor abandoning to the borrower the profit derivable from his personal employment of the capital lent.

As the investigation of the interest of capital lent will help to throw light on the subject of the profit derivable from its personal employment, it may be as well, in the first instance, to acquire a just idea of the nature and variation of interest.