IN GEOMETRIC TERMS

the number of Willingness lines for any one Individualis infinite, and every other individual will have his ownfamily of lines.

We conceive, then, of a map for Individual 1 alone,covered with a family of such Willingness lines infinite innumber, arranged so as to vary gradually from each tothe next, like the lines of elevation on a geographic con-tour map of a mountain. 5

While Individual 1 finds all points on any one Willing-ness line of equal desirability or “wantability,” he wouldrather have his income position on lines more to the“northeast,” or farther from the origin. Each Willingnessline might be labelled with a number representing speci-fically the total desirability or “wantability” pertaining toeach and all of the income positions on it. It is the locusor assemblage of these points (combinations of incomefor the two years) equally desirable in the estimation ofIndividual 1 . A greater aggregate income in the two yearsmay be offset in respect to the resulting total desirabilityby a less convenient distribution between the two yearsand vice versa. 6

But we are not yet interested in such differences of levelor total desirability between the Willingness lines. Weare here interested only in the directions of the Willing-ness lines at different points, representing the different

“ Those familiar with a contour map will find the analogy a good one,since each Willingness line represents a level of desirability differentfrom the others, the level or height being here conceived as measuredin the third dimension, that is, at right angles to the page of the map.

“It would, of course, be possible to present the Willingness lines interms of total desirability or wantability without supposing anyhypothetical borrowing or lending; this was done in the Rate of Inter-est (Appendix to Chapter VII). The Willingness lines were there callediso-desirability lines. They might also be called lines of indifference.

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