Even with targets, the second problem is working out exactly how much quantitative easing is enough. Very simply, whether raising the money in circulation boosts incomes depends also on what economists call the “velocity” of money. If those selling assets to the central bank simply put their spoils on deposit, for example, the potential boost from the increase in money will be tempered. Knowing the velocity of money therefore is crucial. Yet this number is hard to pin down.

The final headache lies in selecting which assets to buy. As the Bank of England showed last week, most central banks go for government bonds. But these tend to be owned by financial institutions, not the struggailing companies and households that need the money most. Besides, government bonds are already super liquid. It would be preferable for central banks to swap cash for harder-to-shift assets such as commercial paper. Another plus would be that purchases of such assets would remove their liquidity discount, giving the likes of the Bank of England at least a fighting chance of recovering their money when things finally recover enough to sell again.