What keeps Mozart modern? A quarter of a millennium since his birth on 27 January 1756, his music is still among the most admired, most valued and most meaningful to us (and, as an inevitable consequence, also among the most abused, exploited and trivialised). Unlike some music of the past that proclaims its distance from ourselves and our cultural context, Mozart's music feels alarmingly close to us, as if it were written yesterday. His operatic characters seem to speak our emotional language, his instruments to invent melodies we can sing. His creative genius leaps across the centuries. How has Mozart kept so in tune with the temper of our times?
Naturally, his enduring appeal is some- thing do with the extraordinary adaptability of his compositions, but it also has some- thing to do with the changing ways in which we have performed them. Of all the revolutions, both scholarly and practical, that have affected our understanding of Mozart's music, none has had such a profound impact as the rediscovery of original instruments (such as the piano Mozart used, which survives in Salzburg) coupled with research into historical performance styles.
Many commentators, including Roger Scruton, have lambasted the so-called early music movement as an example of misleading historicism that adds nothing to our understanding of the great composers.
"We need a revival of period strings as much as we need a revival of period dentistry," wrote one acerbic critic.
In fact, the historical performance movement has revolutionised our understanding of Mozart and made his work fresh for new audiences.
Only 50 years ago Mozart was represented in the concert repertory by a tiny number of works, not all of them typical, too often heard at the beginning of concerts before the orchestra moved on to the serious business of Beethoven and Brahms. The idea of his music as the product of an eternal child, an innocent upbeat to the really profound emotional turbulence of the Romantics, was perpetuated by a performance style that treated him in exactly this way, assimilating his style to the period that followed. To a disturbing extent, orchestras played all music - including Mozart - in their style rather than the style of the period.
That soon began to change, however. The early music movement, pioneered by David Munrow and others, originally took off as a revival of forgotten instruments to play forgotten repertory from Machaut to Monte-verdi, but it soon moved on to the late baroque, revealing Bach and Handel in new colours. The next step was Mozart, and this produced a radical change from 19th-century-style performances to others that showed him as an 18th-century composer growing out of the conventions of the baroque.
The first sign of this was the success of Mozart interpretations by chamber orchestras that had been formed mainly for the performance of baroque music. In the 1960s, the English Chamber Orchestra's glittering Mozart performances with Daniel Barenboim and Vladimir Ashkenazy set a new benchmark for crispness, lively articulation and stylishness. The young Colin Davis, Neville Marriner and the Academy of St Martin-in-the-Fields and many others created a fresh new style which matched the demands of listeners - especially in the LP and hi-fi era, when recordings and broadcasting did so much to disseminate music to a wider public.
During the 1970s, however, something else happened. The period-instrument groups that had been set up to play baroque music in the wake of Munrow's success, notably Christopher Hogwood's Academy of Ancient Music and Trevor Pinnock's English Concert, moved on from Bach and Handel to Mozart and Beethoven. I can remember vividly the impact they made: even if one knew little about the actual historical basis of the sounds and playing techniques, the grace, lightness and passion of the playing were disarming. Others clearly felt the same, and not only in this country: in New York, by the start of the 1980s, Hogwood's rational, clean-edged Mozart was up there with Pavarotti.
The creation and reflection of changing taste in performance is a fascinating process, and what happened to Mozart shows, I think, that we had become subliminally dissatisfied with the creamy, sustained sounds of the big symphony orchestras squeezing Mozart out like toothpaste from a tube. The familiar long lines in the strings, sustained wind chords and deftly moulded textures one heard from the Philharmonic orchestras of Vienna or Berlin, conducted by Karl Bohm or Herbert von Karajan, seemed increasingly irrelevant. This was not just a matter of historical accuracy; it was a matter of taste. Yet it was a taste that seemed to suit the times, and - just as Neville Marriner and Raymond Leppard had created an admirable sound for the LP era - once the CD, with its premium on absolute clarity and transparency of texture, came along in the 1980s, it was the period-instrument bands of Hogwood, Pinnock, Roger Norrington and John Eliot Gardiner that carried the day.
The revolution was soon being applied to Mozart's wonderfully vivid operatic writing. Nikolaus Harnoncourt used a mixture of modern instruments and old performing styles to create a hugely influential Idomeneo, full of fierce accents and brilliant colours, in Zurich in 1980. Then the much quieter and reserved Arnold Ostman created the great Mozart-da Ponte operas for the tiny court theatre at Drottningholm, Sweden. These productions were released on CD, enabling a new way of singing and playing to be heard. Simon Rattle insisted that Glyndebourne, at the heart of the English Mozart tradition, use the period instruments of the Orchestra of the Age of Enlightenment for performances of Mozart opera.
All this made Mozart sound new exactly at a time when his reputation was being given a seismic shift by the film Ama- deus. This extravaganza (rather than the superbly economical stage play of the same name) created a vision of Mozart as our contemporary: a giggling, foul-mouthed youth who was still able to create music of utter beauty.
Yet what the performance revolution told us about Mozart was something equally important and true: that he was not a vague and disengaged genius wri-ting music for posterity. He was a highly practical musician writing music for the instruments of his time, for the performers and singers he loved and revered and drank with and played with and had fun with, and that the best service to his genius was to try to recapture something of that sound-world when approaching his music today.
What is happening in performance now is supremely ironic: those orchestras that clung longest to the Romantic style of performing Mozart's music are rushing to get up to date with the best of "old" playing. So, in Salzburg for Mozart's 250th birthday it will be Harnoncourt, among others, conducting the Vienna Philharmonic. The Berlin Philharmonic has already appointed Rattle to bring stylistic variety and vigour to its music-making.
Other conductors are absorbing period style, consciously or unconsciously, and at the same time early-instrument groups are becoming much freer in their use of historical evidence. It's a fascinating melting pot, and I suspect it would have delighted a composer who knew the supreme worth of what he was creating.
Nicholas Kenyon is director of the BBC Proms and the author of The Faber Pocket Guide to Mozart (£8.99)