IN GEOMETRIC TERMS

lines represent subjective conditions; the Opportunityline represents objective conditions. The 0 line of anindividual is simply one curve, while the W line is oneof many. There is some rate of time preference repre-sented by the angle or slope of a IF line on the charts, beit positive, negative, or zero, corresponding to everypossible income position of an individual wherever on thechart it may be. But this is not true of the 0 line. Thereis only a limited region of options on the map, boundedby a single curve.

As already explained, if the opportunity area enclosedby the 0 line shrinks to a single point, there is no deter-minate tangent and we automatically revert to the firstapproximation in which there is no opportunity to choosefrom among options.

Thus the investment opportunity influence may, theo-retically at least, vanish entirely and lead us back to thefirst approximation, but the impatience influence cannever vanish. Practically however, investment opportu-nity never quite vanishes. There is always at least someflexibility in everybody’s income, but in primitive so-ciety, the range of opportunity is relatively small. Whilethe Opportunity line never entirely collapses into a math-ematical point, yet, for a person in primitive society itis an almost negligible spot or ring and could exert onlya negligible influence on the rate of interest, even if itwere to double in diameter or were to change in form.In such a society the only important influences on therate of interest must come largely from a change in themap, that is, in the distribution of impatience relativelyto income.

But when, as in modern society, the range of invest-ment opportunity is great, the slopes of the Opportunity

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