It seems at first to be a puzzling scenario, and you might wonder whether it is possible at all: output can be at potential?but still not be sustainable. Yet a chapter of the International Monetary Fund’s latest World Economic Outlook illuminates just this scenario. We may even be living in it.

Output is “at potential” when it does not generate inflationary or deflationary pressure. Sustainability — and I am referring here to financial sustainability, not the environmental kind — is something else entirely. Output is financially sustainable when spending patterns and the distribution of income are such that the fruit of economic activity can be absorbed without creating dangerous imbalances in the financial system. It is unsustainable if generating enough demand to absorb the output of the economy requires too much borrowing, real rates of interest rates that are far below zero, or both.

To see how that predicament might arise, start by imagining an economy that is balanced in the sense that the amount of money which households and businesses wish to save is exactly the same as the amount they wished to spend on physical investments. So far, so good. But suppose growth of potential output then fell sharply. The level of desired investment would also fall, because the needed capital stock would be smaller. But the amount that people wished to save might not fall, or not by as much; in fact, if people expect to be poorer in future, they might even wish to save more. If so, real interest rates might need to decline sharply, to restore balance between investment and savings.