John Stuart Mill,
Principles of Political Economy
“ There are two modes in which the Cost of Labor, which is correctly represented (money being supposed invariable as well as efficiency) by the money wages of the laborer, may be increased. The laborer may obtain greater comforts; wages in kind—real wages—may rise. Or the progress of population may force down cultivation to inferior soils and more costly processes; thus raising the cost of production, the value, and the price, of the chief articles of the laborer's consumption. ”
