I was talking to my friend R., an attorney, about 'Writing is thinking.'
She says, Absolutely, writing is thinking: if you can't explain something in writing, you don't understand it.
I believe that myself* and have experienced it many times. For me, writing is a diagnostic. When I discover that I can't put a concept or argument into words, I realize that I don't understand the concept. I don't know what I'm talking about, literally.
But I didn't know that going in.
* For verbal disciplines -- this idea has been wildly over-generalized to math.