The Big Question: Is it right to complete a work of art left unfinished by its creator?

Why are we asking this now?

Because a Russian composer called Anton Safronov has completed a version of Schubert's Symphony No 8 in B Minor – the "Unfinished" Symphony, abandoned by the German composer in 1822. It was first performed at the Festival Hall, London on Tuesday night, conducted by Vladimir Jurowski.

How do we know it's unfinished?

Several reasons. Only two movements were fully orchestrated by Schubert, whereas symphonies generally unfold over four. A third movement is hinted at in a scherzo (or sketch) for piano, but only two pages were written for orchestra. Some musicologists are sure that an entr'acte he wrote for the incidental music to Rosamunde was originally conceived as the fourth movement.

So why didn't he finish it in 1822?

The generally accepted explanation is that he ran into trouble trying to compose in B minor – a hellishly difficult key for valveless brass instruments, like horns and trumpets.

What did Safronov bring to the work?

He completed the third movement, based on the scherzo, and created the fourth movement himself, based on themes in Schubert's piano works.

Are there many unfinished musical works in existence?

Lots. Only two movements of Mahler's Symphony No 10 were orchestrated at his death, leaving many composers keen to fill out the texture of the outline score of the rest. Bach's The Art of Fugue stopped dead when he did – the music ceases on the page – and reconstructions and completions were still being written in the 1990s.

Debate has raged since 1791 as to which parts of Mozart's Requiem Mass in D Minor were left unfinished at his death, but were later touched in by Franz Sussmayr. Bruckner's 9th was never finished. Elgar's Symphony No 3 was interrupted by death, but he left sketches which were worked into a playable reconstruction by Anthony Payne. Puccini popped his clogs before he could finish Turandot, while Alban Berg's Lulu – two acts long at his death in 1935 – had to wait until 1979 to achieve musical "closure."

Were these various 'completions' successful?

Some were laughed at by aficionados of the composer. Poor Geoffrey McNeely, in 1991, was ridiculed for the ineptness of his attempt to complete the Mozart Requiem, although others had seen their attempts similarly scorned. It seems that the closer a modern composer sticks to the master's musical sketches and try-outs in finishing a piece of music (such as Payne's faithful bandaging-together of Elgar's Third) the more successful he will be. It comes down to an elaborate game of Call My Bluff, in which audiences are invited to identify which bars are genuine Mozart (or Elgar or Schubert) and which are by an upstart pasticheur.

Do unfinished masterpieces exist in other art forms?

They're all over the place. Literature is full of not-quite-endings. Dickens had written only six of the 12 chapters of The Mystery of Edwin Drood when he died. Chaucer's Canterbury Tales was supposed to be longer (only 23 of the 29 pilgrims actually tell tales) while Wordsworth's immense The Prelude was originally supposed to be just that – the preface to a much longer work called The Excursion.

Byron was on the 17th canto of his masterpiece Don Juan when he died in Greece. Spencer's The Faerie Queen, the longest epic poem in English, was designed to be twice as long. The 100 novels of Balzac's La Comedie Humaine were meant to be 148. Hemingway and Scott Fitzgerald died leaving incomplete manuscripts.

Renaissance artists were famously relaxed about completing each other's work. It was a sign of the mentor-protégé bond that existed between them that Giulio Romano was allowed to finish off Raphael's "Transfiguration", and Titian empowered to put the finishing touches to Giorgione's "Sleeping Venus".

In sculptor circles, the incomplete artwork had its own integrity: Donatello pioneered the "non finito" technique, in which he sculpted figures only partly out of the stone or marble – leaving them looking as if there were stuck in the block from which they emerged. And let us not forget Gaudi's Sagrada Famiglia cathedral in Barcelona, on which work started in the 1880s and still continues today.

And are attempts made to finish them too?

Certainly, with varying degrees of success. Many crime writers have tried to finish Edwin Drood on Dickens's behalf, without knowing whom he had in mind as the murderer: there was a film, and even a stage musical, neither terribly good. Nobody ever tried to finish Don Juan or The Canterbury Tales – or, thank God, The Excursion – but the estate of JRR Tolkien did well for themselves by publishing definitive versions (culled from incomplete notes) of the great man's The Silmarillion, The History of Middle-Earth and, most recently, The Children of Hurin.

Nobody has had the nerve to improve on Donatello's "unfinished" figures. But the work on the Sagrada Familia continues. It is, remarkably, still in the act of being created first time round...

Are 'completions' of artworks morally or aesthetically doubtful?

Unlike the writing of "sequels" to famous works of literature, which tend to be dismissed as opportunistic raids on the wallets of Jane Austen or Daphne Du Maurier fans, musical "completions" are often regarded benignly. They are invariably the work of lifelong students of the composer – amateurs in the best sense, lovers of the oeuvre, familiar with every note, drenched in his idioms, sensitive to every tonal shift in sonata, scherzo and symphony. If they can bring a key composition to a satisfying conclusion, without too many egregiously false notes, which music lover could complain?

On the other hand, some see this dabbling as, at best, a monumental waste of time, and, at worst, a cheap attempt to hitch a ride to immortality by twinning one's name with one of the Olympians.

Sometimes we're content with fragments. Would we thank any poet, however well-intentioned, for "mending" literature's greatest fragment, Coleridge's Kubla Khan? Would we find "The Winged Victory of Samothrace" in the Louvre more thrilling if someone gave her a lovely new head? No. The incomplete artwork, like the interrupted cadence, is a thing of tantalising mystery. It endures in its own Ozymandian dignity.

So should we leave these works well alone?

Yes...

* It's pointless to second-guess an artist's intentions

* It's impertinent to meddle with the work of a genius

* Nobody has ever 'completed' an artwork to the satisfaction of academics or critics – so why bother?

No...

* Human nature abhors the unfinished, the half-drawn, the inchoate

* Art is not Holy Writ – why not try to improve what is incomplete?

* We can teach the young more about art if we give them the 'whole picture' of what the artist/composer probably intended