Edvard Munch’s The Scream, from 1893, is one of the world’s most famous paintings, but for years art historians have mostly ignored a tiny inscription, written in pencil, at the upper left corner of its frame, reading: “Could only have been painted by a madman”.

Who wrote the sentence there? Some thought a disgruntled viewer might have vandalised the work while it was in a gallery; others imagined it was the artist himself who had jotted the enigmatic sentence. But then why?