"The bursting of this bubble will be across all countries and all assets, with the probable exception of high-grade bonds," Grantham warned. "Since no similar global event has occurred before, the stresses to the system are likely to be unexpected. All of this is likely to depress confidence and lower economic activity." Ouch. Grantham sees two big potential catalysts that might turn this bull market into a bear: a surge in inflation, leading to higher interest rates, and a squeeze on profit margins, which are currently running way above long-term averages. As for timing, he concedes that's impossible to predict. But here's the kicker: Even Grantham thinks you probably need to be bullish right now. The reason? Most bubbles, he notes, go through a short but dramatic "exponential phase" just before they burst. Like Japan in 1989 or the Internet in early 2000. "My colleagues," wrote Grantham, "suggest that this global bubble has not yet had this phase and perhaps they are right. ... In which case, pessimists or conservatives will take considerably more pain."