The Wealth of Nations by Part 1 Chapter 11 Page 63

increases with the increasing quantity of food, or with the growing improvement and cultivation of the lands; and as the nature of their business admits of the utmost subdivisions of labour, the quantity of materials which they can work up increases in a much greater proportion than their numbers. Hence arises a demand for every sort of material which human invention can employ, either usefully or ornamentally, in building, dress, equipage, or household furniture; for the fossils and minerals contained in the bowels of the earth; the precious metals, and the precious stones.

Food is in this manner not only the original source of rent, but every other part of the produce of land which afterwards affords rent derives that part of its value from the improvement of the powers of labour in producing food by means of the improvement and cultivation of land.