From Margin of Safety:
One of the recurrent themes of this book is that the future is unpredictable. No one knows whether the economy will shrink or grow (or how fast), what the rate of inflation will be, and whether interest rates and share prices will rise or fall. Investors intent on avoiding loss consequently must position themselves to survive and even prosper under any circumstances. Bad luck can befall you; mistakes happen. The river may overflow its banks only once or twice in a century, but you still buy flood insurance on your house each year. Similarly we may only have one or two economic depressions or financial panics in a century and hyperinflation may never ruin the U.S. economy, but the prudent, farsighted investor manages his or her portfolio with the knowledge that financial catastrophes can and do occur. Investors must be willing to forego some near-term return, if necessary, as an insurance premium against unexpected and unpredictable adversity.
Choosing to avoid loss is not a complete investment strategy; it says nothing about what to buy and sell, about which risks are acceptable and which are not. A loss-avoidance strategy does not mean that investors should hold all or even half of their portfolios in U.S. Treasury bills or own sizable caches of gold bullion. Rather, investors must be aware that the world can change unexpectedly and sometimes dramatically; the future may be very different from the present or recent past. Investors must be prepared for any eventuality.