Just a thought regarding the discussion surrounding P/E calculation: it appears you folks are comparing apples and oranges.
The most common usage of P/E is with regards to individual stocks and is the quotient of the stock's share price and earnings per share.
However, Ghost is referring to the P/E of a market, in this case the S&P 500. That calculation uses the index value instead of a stock's share price.
Two different things.