In an effort to avoid the normative connotations of the word "natural," James Tobin (following the lead of Franco Modigliani), introduced the term the “Non-Accelerating Inflation Rate of Unemployment” (NAIRU), which corresponds to the situation where the real gross domestic product equals potential output. It has been called the "inflation threshold" unemployment rate or the inflation barrier. This concept is identical to Milton Friedman’s concept of the "natural" rate but reflects the fact that there is nothing "natural" about an economy. The level of the NAIRU depends on the degree of "supply side" unemployment, i.e., joblessness that can't be abolished by high demand. This includes frictional, mismatch, and Classical unemployment. When the actual unemployment rate equals the NAIRU, there is no cyclical or deficient-demand unemployment. That is, Keynes’ involuntary unemployment does not exist.