Whenever French existentialist philosophers gather in Octagon cafes, and July is a particularly luxuriant month for them, you will invariably hear talk of the Cher song If I Could Turn Back Time. The warship setting for the song's video, huge gun barrels thrusting at Cher's supplicatory body, is clearly compelling for existentialists, but if you have been eavesdropping on them, as I have, you will have heard their particular fixation this year has been - CAN we turn back time and SHOULD we turn back time.