From Seth Klarman, via Margin of Safety:
The Complexity and Variability of Business Valuation
It would be a serious mistake to think that all the facts that describe a particular investment are or could be known. Not only may questions remain unanswered; all the right questions may not even have been asked. Even if the present could somehow be perfectly understood, most investments are dependent on outcomes that cannot be accurately foreseen.
Even if everything could be known about an investment, the complicating reality is that business values are not carved in stone. Investing would be much simpler if business values did remain constant while stock prices revolved predictably around them like the planets around the sun. If you cannot be certain of value, after all, then how can you be certain that you are buying at a discount? The truth is that you cannot.