Are Artificially Low Interest Rates Producing Artificially High Asset Prices?

The popular media are full of stories about the sins being committed by central banks.  They claim that central banks are keeping interest rates "artificially" low and that is leading to "artificially" high asset prices.  Paul Krugman explains why these claims are nonsense. 

In the first place, the economics profession does not recognize the concept of artificially high interest rates.  It uses the concept of the natural rate of interest which holds that the rate of interest that does not lead to inflation is the natural rate of interest.  Since there is little indication of inflation in nations where central bank policy rates are low, our current interest rates are at the level that we should expect with low inflation.  Central banks have helped to set inflation expectations through their policies and the market expects inflation rates to be low.  Investors are willing to pay higher prices for safe assets as long as they expect inflation to remain low.

Stock prices have also risen during this period of low interest rates and low inflation.  Does that mean that stock prices are "artificially" high?  Corporate profits have also been high and the price to earnings ratio is close to its long term average.  Market fundamentals seem to explain the rise in stock prices.  It does not seem that stock investors have been taking on excessive risks because interest rates are too low.