To measure is to determine the ratio of one quantity to another which is invariable or assumed to be invariable. Invariability in respect of the property to be measured, or at least the legitimacy of assuming such invariability, is a sine qua non of all measurement. Only when this assumption is admissible is it possible to determine the variations that are to be measured.
Ludwig von Mises, The Theory of Money and Credit
capitalism, economics, freedom, money