Cassel first criticizes the theorists of marginal utility on the grounds that it is impossible to measure in real terms and make a direct comparison between the various needs either of a single person or of a number of people compared with one another. For any measurement, he claims, demands a unit of measurement, and the theorists of marginal utility have never established and can never establish a unit of this kind. Rather, we only gain a measurable sign of psychological processes, of the varying intensity of our feelings, by observing some kind of outward effect they have, i.e. in the case in question, by evaluating the goods to be bought or sold in terms of some conventional yard-stick, most simply in monetary terms. ‘In money’, says Cassel, ‘the individual possesses a scale of value by the aid of which he is able not only to classify his needs, but also to express their relative intensity in numerical terms. If need be (i.e. if I cannot get it more cheaply) I am prepared to pay 10 marks (but no more) for a certain good. For another good I might perhaps pay up to 20 marks. This means that not only is the second good more important to me than the first, it is also, over and above that, precisely twice as important.’ In passing, in conventional linguistic usage this is only true when relatively small portions of my assets or income are involved-in other cases it is false. For example, I should if necessary spend half my income on accommodation

and dress appropriate to my social class, assuming the prices of other goods remain unchanged; for food I cannot, of course, spend more than my whole income, even if I need to. Does that mean that food, for me, is only twice as important as accommodation and dress appropriate to my social class? ‘Thus’, Cassel continues, ‘money is a scale of value for the individual, and by means of trade it becomes a shared, public scale of value, too. For if A and B are prepared at any time to exchange their units of value, the one mark coin, for the same goods, this proves that A’s and B’s scales of value are in fact identical…. It is of course impossible a priori to compare the intensity of A’s needs with B’s. At least such a comparison lies completely outside the domain of economics. But if I make the assumption that A’s and B’s needs are of equal intensity as soon as they both evaluate these needs at the price of one mark, then I have derived from the psychological presuppositions all that is of significance for the economic side of the matter.’