It will be observed that the landowner's share of the product, or economic rent, is not identical with commercial rent. The latter is a payment for land and capital, or land and improvements, combined. When a man pays nine hundred dollars for the use of a house and lot for a year, this sum contains two elements, economic rent for the lot, and interest on the money invested in the house. Assuming that the house is worth ten thousand dollars, and that the usual return on such investments is eight per cent., we see that eight hundred dollars goes to the owner as interest on his capital, and only one hundred dollars as rent for his land. Similarly the price paid by a tenant for the use of an improved farm is partly interest on the value of the improvements, and partly economic rent. In both cases the owner may reckon the land as so much capital value, and the economic rent as interest thereon, just as the commercial rent for the buildings and other improvements is interest on their capital value; but the economist distinguishes between them because he knows that they are determined by different forces, and that the distinction is of importance. He knows, for example, that the supply of land is fixed, while the supply of capital is capable of indefinite increase. In many situations, therefore, rent increases, but interest remains stationary or declines. Sometimes, though more rarely, the reverse occurs. As we shall see later, this and some other specific characteristics of land and rent have important moral aspects; consequently the moralist cannot afford to confuse rent with interest. _The Cause of Economic Rent_