Commentary: ‘When the facts change, I change my mind. What do you do, sir?’ This familiar question, usually attributed, apocryphally, to Keynes or Churchill, was alluded to by the British Prime Minister in January 2021 to justify a third national lockdown when he announced: ‘The facts are changing and we must change our response’. The attraction of such words is clear: they offer a seemingly indisputable logic for changing course when this becomes convenient, heading off the perennial accusations of ‘U-turns’ whenever an earlier policy is being discarded.