If you look at Schumann's Carnaval op 9 the piece following Coquette is "Sphinxs" (page 11 Kalmus vol 2) and it consists of 3 bars of 4, 3, 4 box shaped notes like in the Scriabin but elongated horizontally. A note says, "The Sphinxes (sic) should not be played."
I have noted Horowitz plays them rather forcefully in his Japan and NYC concert(s). If not "Breves" what are these notes called, or did Schumann invent them? Why would Schumann write unplayable notes at all? Maybe he was a forerunner of John Cage.
in the Humoreske Op 20 there is a section "Hastig" where the composer indicates "this inner part is not to be played. The player is to 'read between the lines' here, as it were." This is yet more mysterious, but I guess a composer can do anything he wants. Elliott Carter. Kaikosru Shapurji Sorabji. Michael Finnissy.
I am an artist (among other things) and I have long thought of hanging a show of, say of 3x5 cards, where paintings would be described in words and not actually painted. Somebody beat me to the punch by selling a sculpture with such a method. Anybody want a symphony described?