### „The general method involved may be very simply stated. In cases where the equilibrium values of our variables can be regarded as the solutions of an extremum (maximum or minimum) problem, it is often possible regardless of the number of variables involved to determine unambiguously the qualitative behavior of our solution values in respect to changes of parameters.“

— Paul A. Samuelson, book Foundations of Economic Analysis

Source: 1940s, Foundations of Economic Analysis, 1947, Ch. 2 : The Theory of Maximizing Behavior