As new fragments come to light, what can the unfinished works tell us about his genius?

Mozart, like Shakespeare, is as popular as ever. Since their respective deaths in 1616 and 1791, each has enjoyed a long, relatively unadulterated seat at the table of Western art, with expanded complete editions of their works materialising every few decades. The audience’s desire to consume the entire output of great creators is only dampened by the publisher’s ability to add anything new. Our reason for surrounding ourselves with everything Mozart or Shakespeare ever penned is simple, as David Cairns has aptly put it: “The better we know their universes – the more we explore its heights and depths – the more marvellous it becomes.”

The difference between Mozart and Shakespeare, however, is the manner in which their complete works have come down to us. Since the 1623 printing of Shakespeare’s Collected Works – referred to by scholars as The First Folio – the general framework of The Bard’s writings has been known. The First Folio contained 36 of the 37 plays (though none of the sonnets), and apart from stylistic edits and added commentary over the centuries, relatively little has changed. Mozart,...