There's nothing you can do that can't be done.
Nothing you can sing that can't be sung.
Nothing you can say but you can learn how to play the game
Why isn't the third line "Nothing you can say that can't be said" ??
Maybe that wouldn't be easy.
It goes to things, words, the difference between them, and language games that we can learn how to play, so adroitly.
Little gets settled on this forum, except more deeply into the minds of the already-convinced. Maybe there is a clue here.