In his spare time, he came up with the Shiller P/E ratio.
The Shiller P/E ratio is calculated as follows: divide the S&P 500 by the average inflation-adjusted earnings from the previous 10 years.
Don't worry about it.
Up is good if you're selling stocks.
Down is good when you're buying.
The chart below was taken from an article at The Daily Reckoning written by Dan Amoss having to do with a speech given March 24, 2010 by Russell Napier of CLSA (no clue) at the CFA Society (Certified Financial Analyst).
Excerpts from this most quotable speech, one of which I've posted below, are all over the place.