With the assistance of mathematics, I can see a property of the ninety-nine dimensional surfaces hidden from the naked eye. If an increase in the price of fertilizer alone always increases the amount the firm buys of caviar, from that fact alone I can predict the answer to the following experiment which I have never seen performed and upon which I have no observations: an increase in the price of caviar alone will increase the amount the firm buys of fertilizer. In thermodynamics such reciprocity or integrability conditions are known as Maxwell Conditions; in economics they are known as Hotelling conditions in honor of Harold Hotelling's 1932 work.

P. 67 - Maximum Principles in Analytical Economics, 1970