Final Degree of Utility
The expression used by Jevons for the degree of utility of the last increment of any commodity secured, or the next increment expected or desired. The increments being regarded as infinitesimal, the degree of utility is not supposed to vary from the last possessed to the next expected. It will be obvious, after a study of the article on Degree of Utility that it is the final degree of utility of various commodities that interests us commercially, not, for instance, their initial or average degrees of utility. That is to say (Fig. 1), if a is a small unit of the commodity A, and b a small unit of the commodity B, and qa the quantity of A I possess, and qb the quantity of B I possess, then, in considering the equivalence of a and b I do not ask whether A or B has the greater initial degree of utility, i.e. I do not compare the lines Oa and Ob, nor do I inquire which has the greater average degree of utility, i.e. I do not compare the height of the rectangle on base Ox which shall equal the area aOxa', with the height of the rectangle on base Oy which shall equal the area bOyb’, but I compare the length xa’ with the length yb’, and ask what are the relative rates at which increments of A and B will now add to my satisfaction. If xa’ is twice the length of yb’, then (since a and b are supposed to be small units, throughout the consumption of which the decline in the curves aa’ bb’ may be neglected) it is obvious that 2b will be equivalent to a, since either increment will yield an equal area of satisfaction.
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