The wood of the forest, the grass of the field, and all the natural fruits of the earth, which when land was in common, cost the laborer only the trouble of gathering them, come, even to him, to have an additional price fixed upon them. He must then pay for the license to gather them, and must give up to the landlord a portion of what his labor either collects or produces. This portion, or what comes to the same thing, the price of this portion, constitutes the rent of the land.
— ADAM SMITH, The Wealth of Nations, Book I., Chap. 6.