Così fan tutte was the first opera I saw when I first came to the US as a schoolboy in the early 1950s. The Metropolitan Opera production was by Lynn Fontanne and Alfred Lunt and, as I recall, was much celebrated as a brilliant yet faithful English-language rendition of a sparkling, beautiful, elegant opera that boasted an excellent cast - John Brownlee as Don Alfonso, Eleanor Steber and Blanche Thebom as the two sisters, Richard Tucker and Frank Guarrero as the young men, Patrice Munsel as Despina - and a fastidiously executed conception as an 18th-century court comedy. I remember a lot of curtsying, many lace hankies, elaborate wigs, acres of beauty spots, much chuckling, and all-around good fun, all of which seemed to go well with the very polished, indeed even superb singing by the ensemble. So powerful was the impression made on me by this Così fan tutte that most of the many subsequent performances of the work that I either saw or listened to seemed variations of that quintessentially classical production.

The intrigue in Così fan tutte is the result of a bet between Alfonso, Ferrando and Guglielmo, inspired neither by a sense of moral purpose nor by ideological passion. Ferrando is in love with Dorabella, Guglielmo with Fiordiligi; Alfonso bets that the women will be unfaithful. A subterfuge is then enacted: the two men pretend that they have been called off to war, then come back in disguise and woo the girls. As Albanian (ie, Oriental) men, the two attempt to seduce each other's fiancée. Guglielmo quickly succeeds with Dorabella; Ferrando needs more time, but he too is successful with Fiordiligi, who is clearly the more serious of the two sisters. Alfonso is helped in the plot by Despina, a cynical maid who assists in her mistresses' downfall, although she does not know of the bet between the men. Finally, the plot is exposed and the women are furious, but they return to their lovers - though Mozart does not specify exactly whether the pairs remain as they were at the outset.

We need to look at Così fan tutte as an opera whose strange lightheartedness hides, or at least underplays, an inner system that is quite severe and amoral in its workings. I do not at all want to say that the work must not be enjoyed as the brilliant romp that in many ways it surely is. But of the three Da Ponte operas, Così fan tutte is not only the last and, in my opinion, the most complex and eccentric, but also the most internally well organised, the most full of echoes and references, and the most difficult to unlock, precisely because it goes further toward the limits of acceptable, ordinary experiences of love, life, and ideas than either of its two immediate predecessors. The reasons for this, and indeed for Così's opacity and even resistance to the kind of political and intellectual interpretive analysis that Figaro and Don Giovanni generally permit, are partly to be found in Mozart's life and times in 1789-90, while he was at work on Così. But hey are also to be found in the way Mozart and Da Ponte created the work together, without a well-known play or a legendary figure to provide them with a framework and directions. Così is the result of a collaboration, and its dynamics, the symmetrical structure of its plot, and the echoic quality of much of its music are internal as well as necessary to its composition, not imported into or imposed on it by an outside source.

Many of the numbers of act one, for example, were written by Mozart to emphasise how the characters think, act and sing in pairs; their lines generally imitate one another and recollect lines sung earlier. Mozart seems to have wanted us to feel we are inside a closed system in which melody, imitation, and parody are very difficult to separate from one another. This is superbly in evidence in the act one sextet, which enacts a sort of miniplay in which Alfonso draws Despina, then the two disguised men, then the two women into his plot, all the while commenting on the action, as he also allows Despina to comment. The whole number (written in the opera's basic key of C major) is a dizzying maze of advance and expostulation, statement, echo, and inversion that rivals anything Mozart ever wrote. It simply sweeps aside the last trace of any sense of stability and gravity that we have so far been able to hold on to.

Yet to encounter Così today, either on disc or in the theatre, is with few exceptions to risk missing how carefully Mozart intended all of this. Opera is experienced in the theatre as a basically undramatic, albeit theatrical and extravagant, form. Most spectators do not understand the language, and if they do they cannot understand the singers; in addition, Così has an aggressively inconsequential plot that is enacted by characters who seem to have no interesting past to unravel or expose, and no encumbering relationships that claim their loyalty and the investment of their emotions. Surface seems to be all, except for the music, which is dazzling.

To come to terms with Così is first of all to be reminded that when it was first performed in Vienna, it was a contemporary opera, not a "classic," as it has become. Mozart worked at it during the first part of 1789, at a time when he had just passed through a period of great difficulty. Figaro and Don Giovanni belong to the same group as Così, of course, but whereas they are expansive, explicit, and intellectually and morally transparent, Così is concentrated, full of implicit and internalised characteristics, and morally and politically limited, if not opaque: the third Da Ponte opera is also, relatively speaking, a late work, rather than just a mature one as its predecessors were. We know very little about these figures; no traces of a former life adhere to them (unlike the characters in Figaro and Don Giovanni, who are steeped in earlier episodes, entanglements, and intrigues); their identities exist in order to be tested and exercised as lovers, and once they have gone through one full turn so that they become the opposite of what they were, the opera ends.

The overture, with its busy, clattering, round-like themes, catches this spirit quite perfectly. Remember that Mozart wrote it after he had finished most of the main body of the opera, after the schematic character of what he was elaborating had impressed itself on his mind. The conclusion of Così is really twofold: this is the way things are because that is what they do - Così fan tutte - and second, they will be like that, one situation, one substitution succeeding another, until by implication the process is stopped by death. All are the same, Così fan tutte, in the meantime. As Fiordiligi says: "E potrà la morte sola far che cangi affetto il cor." Death takes the place of Christian reconciliation and redemption, the key to our true, if unknown and indescribable, hope of rest and stability, soothing and consoling without providing anything more than a theoretical intimation of final repose.

But like nearly every serious subject with which the opera flirts, death is kept at bay, indeed is mostly left out of Così fan tutte. Here we should recall those extraordinary feelings of solitary longing and coldness about which Mozart spoke while he worked on the opera. What affects us about Così is of course the music, which often seems so incongruously more interesting than the situation Mozart uses it for, except when (especially in act two) the four lovers express their complex feelings of elation, regret, fear, and outrage. But even at such moments the disparity between Fiordiligi's assertion of faith and devotion in "Come scoglio" and the genuinely frivolous game she is involved in deflates the noble sentiments and music she utters, making that music seem both impossibly overstated and sensationally beautiful at the same time - a combination, I think, that corresponds to Mozart's feelings of unsatisfied longing and cold mastery. Listening to the aria and seeing the hubbub of serious and comic elements jostling one another on the stage, we are kept from wandering off into either speculation or despair, obligated to follow the tight discipline of Mozart's rigour.

Mozart never ventured closer to the potentially terrifying view he and Da Ponte seem to have uncovered of a universe shorn of any redemptive or palliative scheme, whose one law is motion and instability expressed as the power of libertinage and manipulation, and whose only conclusion is the terminal repose provided by death. That so astonishingly satisfying a musical score should be joined to so heedless and insignificant a tale is what Così fan tutte accomplishes with such unique virtuosity. But we should not, I think, believe that the candid fun of the work does any more than hold its ominous vision in abeyance - that is, for as long as Così fan tutte's limits are not permitted to invade the stage.

· This is an edited extract from the essay Così fan tutte at the Limits, taken from On Late Style by Edward W Said (Bloomsbury). Nicholas Hytner's production of Così fan tutte is at Glyndebourne until July 10, then touring. Box office: 01273 813813 glyndebourne.com. Jonathan Miller's production is at the Royal Opera House, London WC2, July 14-22. Box office: 020 7304 4000 roh.org.uk