Mozart was not at all a purely instinctive, intuitive artist. His remarks to the effect that he “loved to plan works, study, and meditate” and that “he preferred to work slowly and with deliberation” [demonstrate this] …
On one level, Mozart’s musical aesthetic is informed by three fundamental and closely related principles that can be designated Appeal, Propriety, Effect. It is axiomatic for him that music must please; and from this it follows not only that the composer must be reasonably cognizant of (but not pandering to) the taste of his audience, but must ensure that his music not “excite disgust” or even to overstep the bounds of nature, that is, that it must observe propriety. Concern for propriety and naturalness in its turn informed, among other things, Mozart’s insistence that harmonic modulations (changes of key) be executed in a smooth, “natural” manner.
Most striking, however, is Mozart’s high esteem for what he called Effect. Considerations of Effect seem in fact to have been a conscious concern for him when he was engaged in the act of composition. The rationale behind numerous changes and revisions visible in his manuscripts of otherwise perfectly correct and respectable readings can often best be understood as manifestations of the desire to create a “good” (or better) effect. To some extent Effect is synonymous with Originality and testifies a desire to do the unexpected and to avoid the obvious. But the term also implies a concern for substantial dramatic impact and even enhances surface sensuous appeal, at which point this particular aesthetic triangle is closed.
It would be vastly mistaken, however, to conclude from the above that Mozart was a “Formalist”. Quite the contrary: for Mozart’s music was, above all, communication and expression; and what it communicated and expressed was the “thoughts and feelings” of the composer. Opera, clearly enough, had to develop the means of adequately – and effectively – representing the feelings and passions of the dramatic characters. But Mozart was convinced that instrumental music, too, was more than the play of abstract sounds. Indeed, instrumental music was not only capable of communicating feeling … it also possessed, to some significant degree, the capacity of representation – “tone painting” as it would be called by later era.
Marshall, Robert (1995) Mozart Speaks. Schirmer, p. 181-182.