A. A. Cournot wrote in "Mathematical Researches into the theory of Wealth" "...the economic system is a whole in which all of the parts are connected and react on one another. An increase in the income of the producers of commodity A will affect the demand for commodities B, C, etc. and the incomes of their producers, and by their reaction will affect the demand for commodity A." Such complex reactions are evident in general equilibrium theory. Although economic interdependence is a general (and presumably important) concept, almost all of the academic writing on the subject is devoted to the interdependence of nations, where a variety of approaches have been proposed.