Ed Yardeni gives it a try:
Why is [Bill Dudley of the Federal Reserve] so sure that the Fed is so powerful when the CPI inflation rate is so close to zero despite the Fed’s extraordinary efforts to reflate the economy over the past few years? Could it be that while the Fed may be able to control inflation, it can’t do much to stop deflation? Macroeconomists like Mr. Dudley are convinced that inflation is always a monetary phenomenon. I agree that rapidly rising inflation is always a consequence of excessively easy monetary policy. Tight monetary policy, if tough enough, can always lower the inflation rate. However, the deflationary pressures of recent years can be attributed to lots of non-monetary developments including the IT revolution, the rebound in productivity growth, the proliferation of free trade following the end of the Cold War, and the emergence of low-wage emerging countries like China. There’s not much that the Fed can do to stop deflation caused by these forces. Indeed, the Fed shouldn’t even try, since such deflation tends to boost the purchasing power of consumers, which is a much better stimulus program than any reflationary policy promoted by the Fed.