Class of Wants in Economics

When faced with the problem of the value to be attached to one unit of a homogeneous supply, man decides on the basis of the value of the least important use he makes of the units. The same reasoning holds good for the question of increasing the available supply of any commodity by the acquisition of an additional, definite number of units. If we say that the acts of choice do not depend on the value attached to a whole class of wants, but on that attached to the concrete wants in estion, we do not add anything to our knowledge and do not trace it back to some better known or more general knowledge. This mode of speaking in terms of classes of wants becomes intelligible only if