On Monday Jeb Bush — or I guess that’s Jeb!, since he seems to have decided to replace his family name with a punctuation mark — finally made his campaign for the White House official, and gave us a first view of his policy goals. First, he says that if elected he would double America’s rate of economic growth to 4 percent. Second, he would make it possible for every American to lose as much weight as he or she wants, without any need for dieting or exercise.

O.K., he didn’t actually make that second promise. But he might as well have. It would have been just as realistic as promising 4 percent growth, and considerably less irresponsible.

I’ll get to Jeb!onomics in a minute, but first let me tell you about a dirty little secret of economics — namely, that we don’t know very much about how to raise the long-run rate of economic growth. Economists do know how to promote recovery from temporary slumps, even if politicians usually refuse to take their advice. But once the economy is near full employment, further growth depends on raising output per worker. And while there are things that might help make that happen, the truth is that nobody knows how to conjure up rapid productivity gains.

Why, then, would Mr. Bush imagine that he is privy to secrets that have evaded everyone else?

One answer, which is actually kind of funny, is that he believes that the growth in Florida’s economy during his time as governor offers a role model for the nation as a whole. Why is that funny? Because everyone except Mr. Bush knows that, during those years, Florida was booming thanks to the mother of all housing bubbles. When the bubble burst, the state plunged into a deep slump, much worse than that in the nation as a whole. Taking the boom and the slump together, Florida’s longer-term economic performance has, if anything, been slightly worse than the national average.