When talking about stock markets, there are three rules you have to remember. First, the stock market is not the economy. Second, the stock market is not the economy. Third, the stock market is not the economy.

So the market plunge of the past few days might mean nothing at all.

On one side, don’t assume that there was a good reason for the slide (although the fact that the Dow fell 666 points on Friday hints either at satanic forces or at some mystical link with the Kushner family’s bum investment at 666 Fifth Avenue). When stocks crashed in 1987, the economist Robert Shiller carried out a real-time survey of investor motivations; it turned out that the crash was essentially a pure self-fulfilling panic. People weren’t selling because some news item caused them to revise their views about stock values; they sold because they saw that other people were selling.

And on the other side, don’t assume that the stock price decline tells us much about the economic future, either. The great economist Paul Samuelson famously quipped that the stock market had predicted nine of the past five recessions. That 1987 crash, for example, was followed not by a recession, but by solid growth.

Still, market turmoil should make us take a hard look at the economy’s prospects. And what the data say, I’d argue, is that at the very least America is heading for a downshift in its growth rate; the available evidence suggests that growth over the next decade will be something like 1.5 percent a year, not the 3 percent Donald Trump and his minions keep promising.