of money, or increase its "velocity." Admittedly, the division of labor increases trade. The three factors listed under "Systems of payment in the community" also affect trade. To the extent that receipts are frequent, regular, and synchronous with outgo, we have a smoothly working economic system, which facilitates commerce. Finally, density of population enormously increases trade. The concentration of men in cities is essential for modern factory production, and the great cities have necessarily grown up about good harbors, or at strategic points for connecting lines of railroads. It seems almost trivial to insist on so obvious a point, but Fisher seems totally to ignore it, for he says: "We conclude, then, that density of population and rapidity of transportation have tended to increase prices by raising velocities. _Historically this concentration of population in cities has been an important factor in raising prices in the United States._"[218] (P. 88. Italics mine.) This is an astounding proposition. It is not merely that the concentration of population in cities has _tended_ to raise prices through raising velocities. It is a statement that this has been an important historical cause of the actual increase in prices. For Fisher's own theory, if the same cause had tended to increase T,[219] that would have offset the rising V's on the other side of the equation, and left prices little affected. But he sees in the V's an independent cause here, divorces them from their connection with T, and follows his logic fearlessly where it leads. I do not see how one could more strikingly illustrate the essential vice of erecting the V's into causal entities.