What Is The Natural Rate Of Interest?

Tyler Cowen, who is the director of the conservative Mercatus Institute at George Mason University, is unhappy because the Fed has been keeping interest rates too low for his taste.  He claims that current interest rates are below the "natural" rate.  Paul Krugman reminds us that the natural rate of interest  was defined as the rate of interest required to produce full employment.  Since we are not at full employment, the interest rate cannot be below its "natural" rate.  Tyler Cowen must have redefined the natural rate more to his taste.