From Howard Marks’ July 2003 Memo (“The Most Important Thing”):
One thing each market participant has to decide is whether he (or she) does or does not believe in the ability to see into the future: the “I know” school versus the “I don’t know” school. The ramifications of this decision are enormous.
If you know what lies ahead, you’ll feel free to invest aggressively, to concentrate positions in the assets you think will do best, and to actively time the market, moving in and out of asset classes as your opinion of their prospects waxes and wanes. If you feel the future isn’t knowable, on the other hand, you’ll invest defensively, acting to avoid losses rather than maximize gains, diversifying more thoroughly, and eschewing efforts at adroit timing.
Of course, I feel strongly that the latter course is the right one. I don’t think many people know more than the consensus about the future of economies and markets. I don’t think markets will ever cease to surprise, or thus that they can be timed. And I think avoiding losses is much more important than pursuing major gains if one is to achieve the absolute prerequisite for investment success: survival.
Book: The Most Important Thing