At the heart of the enigma of Così fan tutte lie changes to the plot far more radical than those attributable to the casting of singers. As one reads through the autograph score, a number of small but quite extraordinary features catch the eye. For example, when Mozart began writing the particella of ‘Rivolgete à me / lui’, he apparently felt unable to enter any pronoun at all and simply left a blank, into which space another hand later added the word ‘lui’. (Fig. 8.) Arguably, this pronoun is the most significant word in the opera up to this point, since it signals the pairings between the disguised suitors and the sisters. Mozart's indecision is thus amazing. In the following analysis, it will be suggested that a wide range of puzzling features in this opera stem from an attempt to overcome a deep-seated flaw in the plot, an attempt that in the end failed to achieve the desired result and was hastily abandoned. The essence of the theory is that Mozart and Da Ponte tried to make the opera work with the disguised men seducing their own women – lovers uncrossed.

Mozart never had any doubt that the composer must be in ultimate control of the libretto for an opera. Writing of a piece by Varesco, he commented that the poet must expect to have to ‘change and rearrange things as much and as often as I want’. The use of the phrase ‘as often’ is significant. It confirms that Mozart's input into a libretto was by no means restricted to a phase of preliminary discussion with his poet. It would continue and perhaps even intensify during the period of composition and rehearsal, when problems with singers and weaknesses in the drama became evident in the theatre. His initial interest lay in the basic outline of the drama, the ‘Plan des stücks’, and he was quite prepared to ask a poet to provide a text for a given plan. Next came the detailed plotting, the working out of the plan. There are useful pointers to Mozart's likely areas of concern in his correspondence with his father during his early career as an opera composer.