Mr. Summers went on to draw a remarkable moral: We have, he suggested, an economy whose normal condition is one of inadequate demand — of at least mild depression — and which only gets anywhere close to full employment when it is being buoyed by bubbles.

I’d weigh in with some further evidence. Look at household debt relative to income. That ratio was roughly stable from 1960 to 1985, but rose rapidly and inexorably from 1985 to 2007, when crisis struck. Yet even with households going ever deeper into debt, the economy’s performance over the period as a whole was mediocre at best, and demand showed no sign of running ahead of supply. Looking forward, we obviously can’t go back to the days of ever-rising debt. Yet that means weaker consumer demand — and without that demand, how are we supposed to return to full employment?

Again, the evidence suggests that we have become an economy whose normal state is one of mild depression, whose brief episodes of prosperity occur only thanks to bubbles and unsustainable borrowing.

Why might this be happening? One answer could be slowing population growth. A growing population creates a demand for new houses, new office buildings, and so on; when growth slows, that demand drops off. America’s working-age population rose rapidly in the 1960s and 1970s, as baby boomers grew up, and its work force rose even faster, as women moved into the labor market. That’s now all behind us. And you can see the effects: Even at the height of the housing bubble, we weren’t building nearly as many houses as in the 1970s.