Very different from this is Ricardo's doctrine of Rent. He makes everything turn on the Cost of Production of the Produce, which is Effort, ignoring the ever-varying demands for the produce, which is Desire. His doctrine, too famous and too long received for us to pass by in this connection, though now superannuated, was for substance, this: there are some lands in every country whose produce just repays the expenses of cultivation, and consequently yields no margin for rent; and the cost of production on these rentless and poorest lands under cultivation, will determine the price of the produce; and as there can be but one price in the same market, the produce raised on more fertile land will be sold for the same price, and this price, besides paying the cost of cultivation, will yield a rent rising higher according as the land is more fertile; so that the rent paid on any land is always a measure of the excess of productiveness of that land over the least productive land under paying cultivation; and therefore, an increased demand for food in consequence of increased population, and the higher price resulting, will force cultivation down upon still poorer soils, or compel a higher culture for less remunerative returns on the old soils, according to the law of diminishing returns, which in either case will raise the rents on all the soils above that grade that just repays the expenses of cultivation; so that it is the sole interest of landlords, as such, that population should be dense and food high, their interest being directly antagonistic to that of the other classes of the community.