While the classic growth companies may continue to generate new sources of earning power, the question is: Can they do this rapidly enough to justify the valuation placed on their current earning power?

One can repeat the question just asked: Is a growth rate triple the potential growth of the economy sustainable indefinitely? If a company can double its earnings over the next 6 years but then needs 10 years before its earnings double again—and perhaps 15 years the next time—then its present P/E ratio will fade with the passage of time. In other words, the price will rise more slowly than the earnings.

And at every moment the investor runs the risk that a change in management, a spry competitor, a shift in customer preferences, or a fundamental economic or social change may slow earning power a lot faster than anticipated. Admittedly, pleasant surprises may come along too, but 40 times earnings already anticipates those. — Peter Bernstein (source)

Financial markets, then, are volatile and unpredictable. Importantly, the markets themselves are far more volatile than the underlying businesses that they represent, which collectively account for their aggregate market capitalization. Put another way, investors are more volatile than investments. Economic reality governs the returns earned by our businesses, and Black Swans are unlikely. But emotions and perceptions — the swings of hope, greed, and fear among the participants in our financial system — govern the returns earned in our markets. Emotional factors magnify or minimize this central core of economic reality, and Black Swans can appear at any time. — John Bogle (source)

Every bubble has two components: an underlying trend that prevails in reality and a misconception relating to that trend. A boom–bust process is set in motion when a trend and a misconception positively reinforce each other. The process is liable to be tested by negative feedback along the way, giving rise to climaxes which may or may not turn out to be genuine. If a trend is strong enough to survive the test, both the trend and the misconception will be further reinforced. Eventually, market expectations become so far removed from reality that people are forced to recognize that a misconception is involved. A twilight period ensues during which doubts grow and more people lose faith, but the prevailing trend is sustained by inertia. As Chuck Prince, former head of Citigroup said during the twilight of the super bubble: ‘As long as the music is playing, you’ve got to get up and dance. We’re still dancing.’ Eventually, a point is reached when the trend is reversed, it then becomes self-reinforcing in the opposite direction. Boom–bust processes tend to be asymmetrical: booms are slow to develop and take a long time to become unsustainable, busts tend to be more abrupt, due to forced liquidation of unsustainable positions and the asymmetries introduced by leverage. — George Soros (source)