APPENDIX TO CHAPTER X - Irving Fisher, The Theory of Interest, as determined by Impatience to Spend Income and Opportunity to Invest it 
The Theory of Interest, as determined by Impatience to Spend Income and Opportunity to Invest it (New York: Macmillan, 1930).
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- Suggestions to Readers
- Part I.: Introduction
- Part I, Chapter I: Income and Capital 2
- Part I, Chapter II: Money Interest and Real Interest
- Part I, Chapter III: Some Common Pitfalls
- Part II.: The Theory In Words
- Part Ii, Chapter IV: Time Preference (human Impatience)
- Part Ii, Chapter V: First Approximation to the Theory of Interest Assuming Each Person's Income Stream Foreknown and Unchangeable Except By Loans
- Part Ii, Chapter VI: Second Approximation to the Theory of Interest Assuming Income Modifiable (1) By Loans and (2) By Other Means
- Part Ii, Chapter VII: The Investment Opportunity Principles
- Part Ii, Chapter VIII: Discussion of the Second Approximation
- Part Ii, Chapter IX: Third Approximation to the Theory of Interest Assuming Income Uncertain
- Part III.: The Theory In Mathematics
- Part Iii, Chapter X: First Approximation In Geometric Terms
- Part Iii, Chapter XI: Second Approximation In Geometric Terms
- Part Iii, Chapter XII: First Approximation In Terms of Formulas
- Part Iii, Chapter XIII: Second Approximation In Terms of Formulas
- Part Iii, Chapter XIV: The Third Approximation Unadapted to Mathematical Formulation
- Part IV.: Further Discussion
- Part Iv, Chapter XV: The Place of Interest In Economics
- Part Iv, Chapter XVI: Relation of Discovery and Invention to Interest Rates
- Part Iv, Chapter XVII: Personal and Business Loans
- Part Iv, Chapter XVIII: Some Illustrative Facts
- Part Iv, Chapter XIX: The Relation of Interest to Money and Prices
- Part Iv, Chapter XX: Objections Considered 73
- Part Iv, Chapter XXI: Summary
- Appendix to Chapter I
- Appendix to Chapter X
- Appendix to Chapter Xii
- Appendix to Chapter Xiii
- Appendix to Chapter Xix
- Appendix to Chapter Xx
APPENDIX TO CHAPTER X
§1 (to Ch. X, § 2)
[Geometric representation of incomes for three years]
IF we proceed from the consideration of two years to that of three, we may still represent our problem geometrically by using a model in three dimensions. Let us imagine three mutually perpendicular axes from an origin O called respectively OX', OX'', OX''', and represent the income combination or income stream for the particular individual by the point P, whose coördinates c', c'', and c''' are the three years' income installments with which the individual is initially endowed. Then through the point P draw, instead of the straight line in the previous representation, a plane ABC cutting the three axes in A,B, and C. This plane has a slope with reference to the two axes OX' and OX'' of equal to 1 + i' (unity or 100 per cent plus the rate of interest connecting the first and second years), and has a slope with reference to the axes OX'' and OX''' represented by equal to 1 + i'' (unity plus the rate of interest connecting the second and third years). Now suppose the space between the axes to be filled with willingness surfaces laminated like the coats of an onion, such that for all points on the same surface, the total desirability or wantability of the triple income combination or income position represented by each of those points will be the same. These surfaces will be such as to approach the three axes and the planes between them, and also such that the attached numbers representing their respective total wantabilities shall increase as they recede from the origin. The plane ABC drawn through P at the slope fixed by the rates of interest just indicated will now be tangent to some one of the willingness surfaces at a point Q, which is the point at which the individual will, under these conditions, fix his income situation, for every point on the plane ABC will have the same present value, and every point on this plane is available to him by borrowing and lending (or buying and selling) at the rates i' and i'', but not all of them will have the same desirability, or wantability. He will select that one which has the maximum wantability, and this will evidently be the point Q, at which the plane is tangent to one of the family of willingness surfaces. This point will be such that the rates of time preference will be equal to the rate of interest.
So much for the individual. The market problem determining the rate of interest is here solved by finding such an orientation for the various planes through the given points called P's as will bring the center of gravity of the tangential points, the Q's, into coincidence with the fixed center of gravity of the P's.
To proceed beyond three years would take us into the fourth dimension and beyond. Such a representation cannot be fully visualized, and therefore has little meaning except to mathematicians.