Irving Fisher (February 27, 1867 - April 29, 1947) was an American economist, statistician, inventor, and Progressive social campaigner. He was one of the earliest American neoclassical economists, though his later work on debt deflation has been embraced by the Post-Keynesian school. Joseph Schumpeter described him as "the greatest economist the United States has ever produced", an assessment later repeated by James Tobin and Milton Friedman. Fisher made important contributions to utility theory and general equilibrium.

Fisher is probably best remembered today in neoclassical economics for his theory of capital, investment, and interest rates, first exposited in his The Nature of Capital and Income (1906) and elaborated on in The Rate of Interest (1907). His 1930 treatise, The Theory of Interest, summed up a lifetime's research into capital, capital budgeting, credit markets, and the factors (including inflation) that determine interest rates. Fisher saw that subjective economic value is not only a function of the amount of goods and services owned or exchanged, but also of the moment in time when they are purchased with money. A good available now has a different value than the same good available at a later date; value has a time as well as a quantity dimension. The relative price of goods available at a future date, in terms of goods sacrificed now, is measured by the interest rate. Fisher made free use of the standard diagrams used to teach undergraduate economics, but labeled the axes "consumption now" and "consumption next period" (instead of the usual schematic alternatives of "apples" and "oranges"). The resulting theory, one of considerable power and insight, was presented in detail in The Theory of Interest (for a concise exposition, see here.) This model, later generalized to the case of K goods and N periods (including the case of infinitely many periods) has become a standard theory of capital and interest, and is described in Gravelle and Rees, and Aliprantis, Brown, and Burkinshaw. This theoretical advance is explained in Hirshleifer.

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