THE THEORY OF INTEREST

the ordinary supply and demand curves and much more.The supply and demand curves, for instance, give us onlythe displacements, or differences in income position, as be-tween P and Q, while the map gives the whole incomeposition of both points. And while we can reconstruct, asabove, the demand and supply curve from our map, wecannot reconstruct our map from the supply and demandcurves.

It may also be noted here that the supply curve is de-rived from the map in spite of the absence, in this firstapproximation, of any investment opportunity or produc-tivity element. The significance of this fact will be moreapparent in Chapter XI where under the second approxi-mation this element is introduced.