So you go out for dinner with a wealthy acquaintance. “I’ll take care of everything,” he says, and orders you a hamburger. Then he orders himself an expensive steak and a bottle of wine, which he doesn’t share. And when the waiter comes with the check, he points at you and says, “Charge it to his credit card.”

Now you understand the essence of the Trump tax cut, signed into law a little over two months ago.

The key thing you need to know is that right now the U.S. government has no business cutting taxes. We need more revenue, not less.

Why? The federal government, as an old line says, is a giant insurance company with an army. Most of its costs come from Social Security, Medicare and Medicaid — and all three programs are becoming more expensive as ever more baby boomers reach retirement age. This means that unless we cut back sharply on benefits that middle-class Americans count on, we will need to raise more revenue than in the past.

Yet even before the tax cut, federal tax receipts were looking weak for an economy with low unemployment and a rising stock market — for example, far lower as a percentage of G.D.P. than the tax take during the Clinton boom of the 1990s, and even a bit lower than they were at the end of the Bush-era expansion. The tax cut will push them lower still. Something will have to give.