The important investment question is how you can estimate true value.

"In the final analysis the stock market is not a voting mechanism but a weighing mechanism."
Benjamin Graham, Security Analysis

Valuation metrics have not changed.

• Eventually, every stock can only be worth the present value of the cash flow it is able to earn for the benefit of investors.  
• In the final analysis, true value will win out.  
• The important investment question is how you can estimate true value.

Markets can be highly efficient even if they make errors.

• Stock valuations depend upon estimations of the earning power of companies many years into the future. Such forecasts are invariably incorrect. 
• Moreover, investment risk is never clearly perceived, so the appropriate rate at which the future should be discounted is never certain. 

Thus, market prices must always be wrong to some extent.

• But at any particular time, it is not obvious to anyone whether they hold only "undervalued" stocks and avoid "overvalued" ones.  
• The fact that the best and the brightest on Wall Street cannot consistently distinguish correct valuations from incorrect ones shows how hard it is to beat the market.