Long and often has it been said that there are only a few basic plots underpinning all the stories we know. Wagner’s Die Meistersinger has the same storyline as the Australian comedy film Strictly Ballroom; Jane Eyre, says Christopher Booker, has the same basic plot as Aladdin; and Ian Fleming’s Doctor No is a regurgitation of the 5,000-year-old Sumerian Epic of Gilgamesh. Dr Johnson said it in the eighteenth century: ‘how small a quantity of real fiction there is in the world; … the same images, with very little variation, have served all the authors who have ever written’. So who can be bothered to count them? Perhaps this is what that pompous pedant Casaubon was up to in Middlemarch as he laboured at his magnum opus, The Key to All Mythologies – a life’s work that was never finished.

In the eighteenth century Carlo Gozzi claimed there were just thirty-six basic stories. Schiller rose to the bait by looking for more but couldn’t even get to thirty-six. In 1920 the French theatre critic Georges Polti tried to prove Gozzi’s case