And here we are given a clue to a fundamental confusion in the quantity theory, a confusion which, accepted by the reader, gives the quantity theory much of its plausibility. I refer to the confusion between _volume of money_, and volume of _money-income_.[344] The two need not be the same. The two generally are not the same. In the case I have described, the one has changed without a change in the other. Now if one wishes to view the process of price-causation from the standpoint of money offered for goods,--an essentially superficial,[345] but frequently useful, view-point--it is clearly money-_income_, rather than mere quantity of money in the country that is important. Into the determination of volume of money-income, however, come factors of a high degree of complexity, among them, prices for which there is no possible place within the confines of so simple and mechanical a doctrine as the quantity theory. In passing, I notice a point to which I called attention in discussing Fisher's factors in the equation of exchange. I refer to his definition of velocity of circulation as the average of "person-turnovers" of money.[346] In the illustration given, there is no reason to suppose that this average is changed. The middlemen simply drop out of the average. They have no money to turn over! But velocity of circulation, defined as "coin-transfer," (_cf._ _supra_, p. 204) has clearly changed. The course of money has been short-circuited. It goes through fewer hands in the course of a given period. This last concept of velocity of circulation is clearly the one that must be used, if the equation of exchange is to be kept straight. But this fact should make it clear that velocity of circulation, instead of being the inflexible thing that Fisher has described, resting in individual habits and practices, a true causal factor in the price making process, is really a highly flexible thing, in large degree a passive function of trade and prices.