Irving Fisher was an American economist, inventor, and 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.Fisher made important contributions to utility theory and general equilibrium. He was also a pioneer in the rigurous study of intertemporal choice in markets, which led him to develop a theory of capital and interest rates. His research on the quantity theory of money inaugurated the school of macroeconomic thought known as "monetarism." Both James Tobin and Milton Friedman called Fisher "the greatest economist the United States has ever produced."Fisher was perhaps the first celebrity economist, but his reputation during his lifetime was irreparably harmed by his public statements, just prior to the Wall Street Crash of 1929, claiming that the stock market had reached "a permanently high plateau." His subsequent theory of debt deflation as an explanation of the Great Depression was largely ignored in favor of the work of John Maynard Keynes. His reputation has since recovered in neoclassical economics, particularly after his work was revived in the late 1950s and more widely due to an increased interest in debt deflation in the Late-2000s recession. Some concepts named after Fisher include the Fisher equation, the Fisher hypothesis, the international Fisher effect, and the Fisher separation theorem.