I discovered my genuine confidence in the sustainability of the current economic growth cycle when I recommended to my 27-year-old Uber driver that he invest some portion of his wages in a diversified index fund. Although the stock market will surely ease off its current pace, it nevertheless should prove far more profitable than standard money market funds with their puny returns. The good news is that the current trend likely will not fizzle out anytime soon thanks to several key factors, including lower taxes and deregulation.

Igniting economic growth, as the Trump administration’s policies are doing, is not as straightforward as it sounds because it is easy to make spectacular mistakes in judgment if caught in the grip of Keynesian economic theory. A day after Barack Obama’s 2008 election, the Dow plunged by almost 500 points. On the day of Trump’s election, the economist Paul Krugman wrote with his legendary overconfidence: “If the question is when markets will recover, a first-pass answer is never.” The Federal Reserve, he added, could not cut rates again to forestall the anticipated recession—and the Trump administration would only make matters worse because it was “ignorant of economic policy.” But the Dow soared by 250 points.

Krugman’s basic mistake is that he wants to use monetary and fiscal policy to shift income and wealth away from investment to consumption, or indeed vice versa. The theory is that only government stimulation can make up for the chronic shortage of private investment, given the general lack of confidence in market institutions. This approach falsely assumes that some omniscient policymaker knows best how to make and implement a collective decision about the appropriate balance between investment and consumption. But there are several errors with this way of thinking.