The flat nine is a cool sounding chord extension. So I have an Eflat 7 flat nine in a song, right after a Cmaj7, etc... The flat nine of E flat would be D. [correction: E]
But those words are meaningless, if you don't know what that sounds like. I myself barely know what this sounds like, because my ears are not that good. I couldn't sing one for you on the spot. I could sing an octave and then up one half step, that's what it is. I couldn't recognize one listening to music.
It seems inadequate, then, to say we think in language. We can certainly use that label for that interval, and make ourselves understood, and understood to our own selves too, writing it down for future reference. But is the manipulation of such signs without understanding their meaning thinking? To really make the flat 9 the object of thought one would have to already be thinking musically, not just manipulating the signs of another system of thought--language.
Words cannot express, we say... But it is a fallacy to think words ever express anything. I could try to evoke this in a poem:
"Ah, the flat nines of Bill Evans make me think of magnolia trees!"
You might get the illusion of understanding here. The language is not really evoking the music, it is just gesturing toward it, and the person reading this line won't figure out what it really sounds like. Words have their own sounds, and I guess those will never sound the same as any flat nine either. Even people who claim that the referent doesn't matter won't read poetry in languages they don't understand semantically. Of course, if we already know what magnolia trees in bloom look and smell like, then we can evoke them in a poem. The reader without this knowledge can substitute a similar kind of memory and go along for the ride.