Assuming... that all available goods of higher order are employed in the most economic fashion, the value of a concrete quantity of a good of higher order is equal to the difference in importance between the satisfactions that can be attained when we have command of the given quantity of the good of higher order whose value we wish to determine and the satisfactions that would be attained if we did not have this quantity at our command.

Menger, Principles, p. 164-5; cited in: Randall G. Holcombe, Great Austrian Economists, p. 90