I think of sound as occupying various physical states, like matter: solid, liquid, vapor, overtones being vapor. As such they cannot be written down; they have simply to be heard and responded to without being destroyed.
It is a feature of Schumann's music that he alternates between a kind of solid music that you recognize--a march, say, or a patriotic song; and music that is completely strange, perhaps because it evokes night or fairy tales. rather more liquid because in-between states. Sometimes he just combines sounds that make no sense except by listening to them within the indicated tempo (usually langsam), and dynamics, including the specified pedaling.
This sensation of half-heard always felt threatening to me and I would avoid it like the plague until I learned to listen to my students' intentions rather than to results that I felt compelled to have them obtain.
I now realize that the printed symbol is to blame, for it leads us to believe that every sound so indicated is equally solid.
