The error of optimism dies in the crisis, but in dying it gives birth to an error of pessimism. This new error is born not an infant, but a giant; for (the) boom has necessarily been a period of strong emotional excitement, and an excited man passes from one form of excitement to another more rapidly than he passes to quiescence.
~ Arthur Cecil Pigou (1877-1959)
For investors, this statement can be interpreted as meaning future returns for every new dollar invested will be lower in times of euphoria than in times of pessimism. These episodes of pessimism are times of great opportunity for the long-term investor. March 2009 will be viewed as the investment opportunity of a lifetime for many, and was a time when many investors made exactly the wrong move of fleeing the stock market to invest in "safe" T-bills or treasuries yielding close to 0%.