We conclude, then, that money must have value to start with, from some source other than the money function, and that there must always be some source of value apart from the money function, if money is to circulate, or to serve as money in other ways. But this is not to assert the doctrine of the commodity school, that its value must arise from the metal of which it is made, or in which it is expected to be redeemed. Nor is it to deny that the money function may add to the original value. On the contrary, the services which money performs are valuable services, and add directly, under conditions which we shall analyze more fully in a later chapter on the functions of money, to the value derived from non-pecuniary sources. Value is not physical, but psychical. And value is not bound up inseparably with labor-pain or marginal utility. CHAPTER VIII THE "EQUATION OF EXCHANGE" In Professor Irving Fisher's _Purchasing Power of Money_[130] we have the most uncompromising and rigorous statement of the quantity theory to be found in modern economic literature. We have, too, a book which follows the logic of the quantity theory more consistently than any other work with which I am acquainted. The book deals with the theory more elaborately and with more detail than any other single volume, and sums up most of what other writers have had to say in defence of the quantity theory. Professor Fisher's book has, moreover, received such enthusiastic recognition from reviewers and others as to justify one in treating it as the "official" exposition of the quantity theory. Thus, Sir David Barbour cites Professor Fisher as the authority on whom he relies for such justification of the theory as may be needed,[131] while Professor A. C. Whitaker declares that he adopts "without qualification the whole body of general monetary theory" for which Professor Fisher stands.[132] Professor J. H. Hollander has recently referred to Professor Fisher's work on money and prices as a model of that combination of theory and inductive verification which constitutes real science.[133] The _American Economic Review_ presents as an annual feature Professor Fisher's "Equation of Exchange."