"Whether a coal mine, for example," he says, "can afford any rent, depends partly upon its fertility, and partly upon its situation. A mine of any kind may be said to be either fertile or barren, according as the quantity of mineral which can brought from it by a certain quantity of labour, is greater or less than what can be brought by an equal quantity from the greater part of other mines of the same kind. Some coal mines, advantageously situated, cannot be wrought on account of their barrenness. The produce does not pay the expense. They can afford neither profit nor rent. There are some, of which the produce is barely sufficient to pay the labour, and replace, together with its ordinary profits, the stock employed in working them. They afford some profit to the undertaker of the work, but no rent to the landlord. They can be wrought advantageously by nobody but the landlord, who being himself the undertaker of the work, gets the ordinary profit of the capital which he employs in it. Many coal mines in Scotland are wrought in this manner, and can be wrought in no other. The landlord will allow nobody else to work them without paying some rent, and nobody can afford to pay any. "Other coal mines in the same country, sufficiently fertile, cannot be wrought on account of their situation. A quantity of mineral sufficient to defray the expense of working, could be brought from the mine by the ordinary, or even less than the ordinary quantity of labour; but in an inland country, thinly inhabited, and without either good roads or water-carriage, this quantity could not be sold." The whole principle of rent is here admirably and perspicuously explained, but every word is as applicable to land as it is to mines; yet he affirms that "it is otherwise in estates above ground. The proportion, both of their produce and of their rent, is in proportion to their absolute, and not to their relative fertility." But suppose that there were no land which did not afford a rent; then, the amount of rent on the worst land would be in proportion to the excess of the value of the produce above the expenditure of capital and the ordinary profits of stock: the same principle would govern the rent of land of a somewhat better quality, or more favourably situated, and therefore the rent of this land would exceed the rent of that inferior to it, by the superior advantages which it possessed; the same might be said of that of the third quality, and so on to the very best. Is it not then as certain that it is the relative fertility of the land which determines the portion of the produce which shall be paid for the rent of land, as it is that the relative fertility of mines determines the portion of their produce, which shall be paid for the rent of mines?