The effects in a Mozart sonata are both slight and far-reaching. Take, for example the sonata in F major K533 which I was listening to yesterday. The first movement is typical of Mozart: a simple figure, childlike in its simplicity, repeated with minor variation, at times with the Alberti bass. The Alberti bass is that left hand accompaniment to the right hand tune, a kind of flowing trill-like repetition that creates a running background to the melody. As you get halfway through any Mozart sonata movement, five minutes or so in, there may be a shift to minor or a tiny development like a wider more yearning kind of interval between a couple of the notes, the effect being the creation of a kind of delicate, domestic crisis within the music. A tiny crisis. I think of a 1780s chambermaid making an error in her sorting of the washing and being rebuked by the Lady or Master of the house. The reaction of the Lady of the house reveals to the chambermaid that she is no longer in favour and will soon be dismissed. A tiny crisis with deep, maybe existential repercussions.
Rousseau was writing his Confessions just a few years earlier in the 1760s. There is a heart-breaking anecdote he recounts from when he was a young man in service and stole a small piece of coloured ribbon, an item of some luxury at the time. The theft was discovered and the young Rousseau planted the ribbon on a middle-aged serving woman who was accused and humiliated, the implication being that as a middle-aged woman it was pathetic for her to be fancifying herself. She was, if I recall rightly, dismissed. Rousseau goes on to confess that this was the act in his life (a life of many guilty acts) of which he was most ashamed. It is a moving anecdote and, much like the habitual thirty second patch of pathos in a movement of a Mozart sonata, the presentation of a trivial crisis with deep consequences. Any period of content can be shattered by the most trivial of incidents. The individual is fragile and vulnerable and can be shattered by as little a thing as a coloured ribbon or a shift from major to minor keys.