Rosina's observation that "everyone knows how to love with their mouth, but not with their heart" is a personal favourite of 'Mozartian' wisdom, surpassed only by Despina's "all men are made of the same pasta".The libretto is genuinely funny, and the humour is contained as much in the Italian text as in the comic situations which it sets up. As such it was ideal material for the prodigious twelve-year-old whose father had brought him to Vienna to show him off to the musical world.

But we cannot overestimate the blow to the child's confidence that must have resulted from the savage hostility with which his completed opera was greeted by the Viennese 'mafia'. His next opera, Bastien und Bastienne, written while the Mozarts were still waiting in Vienna in the hope that La Finta Semplice may yet be put on, was a massively less ambitious work, scored for a very small orchestra and a cast of three.

His next operas were written in and for Italy, and were consequently far more dramatically static, arias often lasting for as long as ten minutes each. Mozart was not to write another comic opera for Vienna until Le Nozze di Figaro in 1786, his first collaboration with Lorenzo Da Ponte, and there are several pre-echoes of that masterpiece in his earlier comedy, most memorably in the ladies' plea for forgiveness in the third act finale.

As the young Mozart sat down in January 1768, though, to start setting Goldoni's text (Coltellini, who adapted the libretto for Mozart and his singers, had the good sense to leave most of Goldoni's lines exactly as he found them, as well as the arrogance to pass the work off as entirely his own!), he must scarcely have been able to believe his own luck.

Certainly there is a feeling of the young composer revelling in all the facilities that had been placed at his disposal, and an infectious playfulness and delight in each dramatic situation with which he is presented. Arias for a drunken Cassandro and a love-lorn Polidoro, a quasi-duel between Cassandro and Fracasso and a scene where Rosina and Cassandro communicate only by signs are all given highly accomplished musical settings, the orchestra portrays a dog barking and yelping, and Cassandro, who pompously and misguidedly prides himself on being particularly well-read and erudite, can only describe the effect Rosina has on him as making his blood go "blo, blo, blo".

No less remarkable are the recitatives, full of scurrying runs in the continuo to underline Cassandro's pomposity, or plaintive chromaticisms to convey Rosina's tears, and constantly exhibiting total harmonic and dramatic assurance. On two or three occasions when lovers are having disagreements, one character will suddenly establish an entirely different key, as if to say "But how can you be in E flat major when I'm in D major - don't you understand me at all?!"

Most extraordinary of all, though, is the warmth and compassion with which the music imbues each character in the story, even the potentially two-dimensional brothers, Cassandro and Polidoro, who end up as real and, despite all their foibles, even endearing people. This was of course one of the hallmarks of Mozart's greatness in his maturity, but it is astonishing to discover to what extent he already possessed this unique gift at the age of twelve.

Polidoro's serenade in Act 2 puts one in mind of the six-year-old Wolfgang proposing marriage to Marie-Antoinette, the future Queen of France, while the music with which Rosina sings of love has a beauty, a depth and a vulnerability which almost defy belief.

Over the past five weeks of rehearsal, everyone involved in this production has become increasingly astonished and inspired by the quality and artistry of this long-neglected work. Each day has witnessed further discovery and further appreciation, and I deeply hope that we shall be able to transmit our admiration and enthusiasm to you this evening. Even if none of us can quite fathom how this could be the work of a twelve-year-old child, it is nonetheless worthy of our wonder, and of our celebration.

The action of the opera takes place within 24 hours in 1768, on a country estate near Cremona, Northern Italy. Fracasso, a captain in the Hungarian army, and his sergeant Simone, have been billetted in the house of two wealthy brothers, Cassandro and Polidoro, and their sister Giacinta. Fracasso is in love with Giacinta, and Simone with her maid, Ninetta, but neither couple can be married without the consent of the two brothers.

We first encounter Cassandro railing against women. Fracasso suggests that his fury might be a cover, and that deep down he's scared of falling in love with Rosina. Cassandro arrogantly scoffs at such a notion, but Fracasso asserts that ultimately women are impossible to resist.

Polidoro pays his respects to Rosina, and proposes marriage within seconds. She, though, asks to be formally courted with visits, gifts and love letters, and as Polidoro never learnt to read or write Ninetta offers to write a letter for him. Polidoro tells a disbelieving Cassandro that Rosina has already fallen for him. Cassandro's defences are immediately undermined when Rosina enters. She knows that her best chance is for Cassandro to feel entirely unintimidated by her, so she adopts a naïve innocence which completely disarms him. She asks for his valuable ring as a keepsake, which he refuses.

Polidoro tells Fracasso to go away so that he can talk to Ninetta about the letter. Fracasso goes to fetch Rosina, and Ninetta advises Polidoro to get on his knees and beg forgiveness. Cassandro enters and wants to know what is going on. Rosina tells him that the letter has been written by her for Cassandro. Meanwhile, Polidoro gives Rosina money as a gift. Cassandro, delighted by the contents of the letter, reluctantly agrees to let Rosina try the ring on, but Simone bursts in with news that Rosina has a visitor. Not wanting to let the ring out of his sight, Cassandro invites everyone to lunch, and there is general rejoicing.

Rosina is worried that she too might be falling in love. She sets Polidoro some more tests, which he again fails with great aplomb. Cassandro, still upset about the ring, is drunk, and he upsets Rosina. She tells him he can only communicate with her through miming. He falls asleep, and she slips the ring back on his finger. Fracasso challenges Cassandro to a duel, but Cassandro eventually escapes. When we next see him, he is taking it out on his little brother.

A new intrigue has been planned. Simone takes Giacinta into hiding, and Fracasso tells the brothers that she has run off with their money. The plot is so successful that Ninetta follows suit, and Simone announces that she too has stolen everything she could find and fled. When the brothers offer Giacinta's and Ninetta's hands in marriage to whoever can find them and bring them back, Rosina reassures them that all will end happily.

During this visit the six-year-old Wolfgang and his sister, Nannerl, played at the Schönbrunn Palace before the Empress Maria Theresa and her family. The Vienna visit was so successful that, after a mere six months at home in Salzburg, the Mozarts set out again on a tour of Europe which was to last three and a half years, incorporating such important musical centres as Mannheim, Paris and London.

While in London Mozart wrote his first symphonies and concert arias, and his father wrote back to Salzburg that "what he [Wolfgang] knew when he left Salzburg is a mere shadow compared with what he knows now. It exceeds all that one can imagine . . . in a word, my boy knows in this his eighth year what one would expect only from a man of forty."

On returning home, Wolfgang encountered his first taste of professional jealousy and distrust - the Salzburg authorities suspected that Mozart's father, Leopold, was producing works and passing them off as his son's compositions, so that in 1767 "the prince of Salzburg, not crediting that such masterly compositions were really those of a child, shut him up for a week, during which he was not permitted to see anyone, and was left only with music paper and the words of an oratorio.

During this time he composed a very capital oratorio [Grabmusik, K.42], which was most highly approved of upon being performed." 1767 also saw the composition of Mozart's first opera, Apollo et Hyacinthus, a short drama in Latin which was given its first performance in the Great Hall of Salzburg University on 13 May.

Leopold Mozart, though, did not want to keep his son isolated in Salzburg for long; "there is a strong rumour that the Mozarts will again not long remain here", wrote one Salzburger in his diary, "but will soon visit the whole of Scandinavia and the whole of Russia, and perhaps even travel to China, which would be a far greater journey and bigger undertaking still". Had they done so the history of music might have been quite different, but as it was, Leopold was planning another visit to Vienna, and on 11 September 1767, less than a year after they returned from their Grand Tour, the Mozarts again left Salzburg.

But Wolfgang's appeal as a child virtuoso was fading, and there were even stirrings of active opposition to the twelve-year-old composer; according to Leopold, the chief maxim of many Viennese musicians "was to avoid most carefully every occasion of seeing us and of admitting little Wolfgang's skill, so that on the many occasions on which they might be asked whether they had heard this boy and what they thought of him, they could always say that they had not heard him and that it could not possibly be true; that it was all humbug and foolishness; that it was all prearranged; that he was given music which he already knew; that it was ridiculous to think that he could compose, and so forth."

Leopold thought about cutting his losses and returning home, but one of his subsequent letters relates how "I first got the idea of having little Wolfgang write an opera from the Emperor himself, when he asked little Wolfgang twice if he would like to compose an opera and conduct it himself. Naturally he accepted, but the Emperor could not say anything else since the opera is Affligio's concern…Now I should not regret any costs, for we will recoup them, if not today, then tomorrow. He who takes no risks, wins nothing."

Giusseppe Affligio (1722-1788) was an international adventurer (Casanova describes him in his memoirs as an elegant and handsome man who nonetheless had the "face of a gallows bird"!), a professional gambler and a theatrical impresario who had recently signed a ten-year contract as "sole holder of the entire franchise for spectacles in Vienna".

As such he was in charge of all opera in the city, and he agreed, seemingly under duress from Leopold, to pay 100 ducats for the opera and to provide a cast of singers from the imperial opera company. Goldoni's comedy La Finta Semplice, which had already been set as an opera in 1764 by Salvatore Perillo in Venice, but which was unknown in Vienna, was chosen as a text, and the libretto was adapted for Wolfgang and his singers by Marco Coltellini, a Tuscan who had moved to Vienna in 1764.

Mozart began composing La Finta Semplice in late January 1768. In March, Leopold Mozart reported that work on the opera was going well, and by June Wolfgang had completed the score of 558 pages. Rehearsals began, selected arias were given private performances in the homes of the Viennese nobility, and Mozart even performed the whole opera on the piano in the house of Baron van Swieten.

At this point, however, and not for the first time in his brief career, intrigue and jealousy set in. Leopold, in a letter dated 30 July 1768, recounts how "all of the composers, Gluck a main protagonist among them, then undermined everything in order to thwart the progress of the opera. The singers were incited, the orchestra was stirred up, and everything was done to stop the performance of the opera.

"The singers, who can hardly read music anyway, should now say that they cannot sing arias which they had previously approved and applauded…The orchestra should now prefer not to be conducted by a boy, and a hundred such things. Then they said that the father, and not the boy, had composed it - but here the slanderers were discredited: I let the next best volume of Metastasio's works to be taken and opened, and presented Wolfgang with the first aria to hand. He picked up the quill and, without hesitating and with the most astonishing speed, set it to music with many instruments, in the presence of many persons of esteem."

Mozart was never paid his 100 ducats, and Affligio even threatened that if a performance of the opera were insisted on, he would see to it that the reception would be completely hostile. Leopold protested by letter to the Emperor on 21 September 1768, but after sixteen fruitless months away from home, they had little choice but to return to Salzburg with the new opera still unperformed, finally leaving Vienna on 29 December 1768. Back in Salzburg, Archbishop Sigismund von Schrattenbach took pity on the Mozarts' rebuffs, and some sort of performance, on an improvised stage, took place in the Archbishop's palace on 1 May 1769. It was the only performance of La Finta Semplice during Mozart's lifetime.

Back in the house, Rosina finally tells Cassandro that she has decided some time ago whom she wants to marry. When Polidoro approaches, she tells Cassandro to hide. She tries to explain to Polidoro that he is in love with love, not with her, before offering her hand not to the expectant Polidoro but to Cassandro. The two of them cruelly taunt the distraught Polidoro.

Giacinta, Fracasso, Ninetta and Simone return to the house demanding immediate weddings. Even now Cassandro tries to renege on his promise, but Rosina keeps him in his place. Giacinta and Ninetta ask for forgiveness, and Rosina exposes her own "innocent deception", saying that if she is not what she made herself out to be Cassandro will nonetheless pardon her. Polidoro is delighted that his brother has been duped, for it means that he is not the only fool, and all ends happily.