Changes in the money supply have no effect on real economic variables such as output, real interest rates and unemployment. If the central bank doubles the money supply, the price level will double too. Twice as many dollars means half as much bang for the buck. This theory, a core belief of classical economics, was first put forward in the 18th century by David Hume. He set out the classical dichotomy that economic variables come in two varieties, nominal and real, and that the things that influence nominal variables do not necessarily affect the real economy. Today few economists think that pure monetary neutrality exists in the real world, at least in the short run. Inflation does affect the real economy because, for instance, there may be sticky prices or money illusion.