Though $3.6 trillion of incremental liquidity has been added to the Fed’s balance sheet since late 2008, nominal GDP was up by just $2.5 trillion from the third quarter of 2008 to the second quarter of this year. As John Maynard Keynes famously pointed out after the Great Depression, when an economy is locked in a “liquidity trap,” with low interest rates unable to induce investment or consumption, attempting to use monetary policy to spur demand is like pushing on a string.