Imagine a computer. Inside it a billion microprocessors run a mathematical system of awesome complexity. Numbers pulse on a hundred screens feeding you alarming but confusing information from the machine. The only controls are three buttons: marked 0bp, 25bp, and 50bp. You must guess the effects of each button and then press the one that will bring calm.

That is what Chairman Ben Bernanke and the governors of the Federal Reserve have to do when they meet on Tuesday to decide whether and by how much to cut US interest rates. Their choice will determine whether credit markets stabilise, whether the US housing downturn becomes a recession and whether inflation is rekindled. It will define Mr Bernanke’s term at the Fed. This is a meeting that matters.