It is well known that the natural rate of interest is difficult to estimate – just think about the challenge of assessing changes in productivity growth. But I think that a comparison with a historical average of past actual rates can give an idea of whether interest rates are out of line.
(2)  Low interest rates underpin the second ingredient, which the recent financial turmoil has brought to everyone’s attention – strong credit growth. The current intellectual debate highlights how standard macroeconomic theories in the Lucasian tradition – based on optimizing agents with rational expectations, complete markets and fully flexible prices – have a hard time assigning credit a relevant role.