The general theory of economic equilibrium was strengthened and made effective as an organon of thought by two powerful subsidiary conceptions — the Margin and Substitution. The notion of the Margin was extended beyond Utility to describe the equilibrium point in given conditions of any economic factor which can be regarded as capable of small variations about a given value, or in its functional relation to a given value.

p. 223 - Essays In Biography (1933) - Alfred Marshall