When the facts change, I change my mind. What do you do sir? That defence of the political u-turn was purportedly uttered by the great economist John Maynard Keynes and is recognised as the classic intellectually respectable case for abandoning a stance in favour of an opposite one. It is certainly the best defence available to Boris Johnson over his apparent volte face on mask-wearing with the new announcement that masks will be compulsory in shops from next week.