It is impossible for people to predict the future perfectly (this is why diversification in investment is so important). "Don't try to time the market" is also

It is impossible for people to predict the future perfectly (this is why diversification in investment is so important). "Don't try to time the market" is also generally considered good advise. It's impossible to know exactly when a crash occurs. That said, it is possible to use market indicators, historical factors, and other numeric qualifications for reasons to be wary, because the investment asset looks too expensive. For instance, up to the 2008 crash, two important indicators -- the price to income ratio, and the price to rent ratio, were very high, indicating real estate was significantly overvalued. The same thing applied to the sky high prices of many .com stocks in 2000, who really didn't have any profit or even cash flow behind it to back the valuation up. There were plenty of experts raising warning flags on these bubbles. Even from a broader perspective, The Economist was wary of Internet stocks in 1999 (https://www.economist.com/node/183809 https://www.economist.com/node/183809) and was wary of the housing bubble that caused the 2008 crash in 2005 (http://www.economist.com/node/4079027 http://www.economist.com/node/4079027). (Of course, The Economist in 1999 used Amazon as the example of an overvalued stock. Whoops. Shows that you "never know". But they wouldn't have been wrong if they had used, say, pets.com.)