Analogous:

The harm that accumulates from trivial piano practicing is mostly not the result of a nefarious purpose but rather of a mindless lack of intent.

I deduct that he didn’t think of the Dark Lord of Evil, Diábolos, Satan, Master of the Four Horsemen, the Destroyer of Worlds, Enemy of all divine and human,

but rather of the Light Lord of Lies, Mephistopheles, Loki, Lucifer, master of might and magic, not maleficent but cunning, the trickster, the shapeshifter, the illusionist, part of the Power that ever schemes Evil, but always creates Good.

I further deduct from Schubert’s vigorous vituperation a virtual pedagogical advice, perhaps his only piano lesson: Sometimes the score cannot be read as it appears on the page, sometimes one has to take out a note or two, sometimes one has to let one hand wash the other, sometimes one has to compress clusters, sometimes the music should sound what it looks like but not be played as it appears on the page. One must grasp what the music should sound like from the page and recreate that expressive idea with one’s own (terrifically trained) hands. One must modify the written score by what could be misunderstood as cheating, but truly is mastering. That’s what Liszt, Rubinstein, Busoni, Rachmaninoff, Hoffmann, Horowitz … to name a few great piano virtuosos … will do! Not to mention building a smaller keyboard with flatter black keys, or modifying the action, or practicing like the devil …

And when asked further, we were told: “It must be natural”. And when we still inquired more, we were given a picturesque metaphor, such as “Imagine a river; your melody must be like a leaf carried by the flow.” or “The lines of the bars in the score are like prison bars: you have to escape them.” or “Imagine carrying the holy grail slowly towards the light.”

At that point we were hesitant to question further, as the impatience of the master increased in proportion to the assumed evidence of our ineptitude. We returned to our practice rooms and tried harder.

But one more persistent quandary would surely have yielded this very practical advice:
“Imagine a rather large pendulum; think of one swing of the weight as one bar; subdivide that movement in four in your mind. Just before the weight comes to a full stop on the other side of the swing, you have to gently catch it and turn it back so it never looses momentum. Seeing it as a physical gesture, it must come from your body; use your arm. Seeing it as a musical gesture, it must come from your emotional intelligence; use your expressive desire. Practically: within a melodic line or a phrase, you have to make each last note of a bar an upbeat to the following one.
|| One, two, three, ääääänd-One, two, three, ääääänd- | (repeat).
This will enable the melody to flow naturally.”

7.1: Whereof one cannot speak, thereof one must create art.