This shows U.S. stocks most expensive since WWII

U.S. stocks are trading at their highest level relative to earnings since WWII, according to one measure that comes from Jim Paulsen of Wells Capital Management.

The median price-earnings ratio of stocks on the New York Stock Exchange is slightly higher than 20 times, more than the readings around 19 hit during bull markets in 2005, 1998 and 1962, according to Paulsen's note to clients Thursday.

And history is on Paulsen's side. The stock market entered a bear market or at least had a correction every time except once when the median NYSE P/E was as high as it is now. (In 2005, the median multiple hit 19 and the stock market didn't enter a bear market until 2008.)

If 2015 does spell the end of the bull market, it certainly wouldn't be a surprise given historical context. With a 200-percent plus gain over more than 2,100 days, this bull ranks as the fourth most powerful and fourth longest since WWII.

At the start of this bull market in 2009, the median NYSE P/E was 12 times trailing 12-month earnings.

The bulls say that valuations are low and can get much richer because P/E's relative to such low interest rates are not at historic highs. Multiples and interest rates (and inflation) do have a long-time relationship.

Historically, the data shows that the S&P 500's P/E could be much higher under low interest rate regimes than high interest rate times, before a selloff would occur.

But the bears say this time is different because rates are not just low—they're at zero. And so even a slight increase in rates on the part of the Fed could expose stocks as overvalued and trigger a bear market, they argue.