Which is not even close to saying that writing is the only medium, and in fact confirms that there are other mediums to think, including just keeping stuff in your head.
Keeping stuff in your head while thinking with the stuff in your head changes it. The same with memories. If you have not noticed this, please challenge the statement: Keeping stuff in your head does not work; you need to externalize in order to use your differential engine, aka brain in order to 'think.'
Having said all this, I think because I am. not the other way 'round! everything else that you may think is thinking, maybe emotional/felt or aesthetic judging, would come from deeper down the stack.
Anyway, tell me about these other media.. and show me how you're not driving complex symbolic insights with them!
its all reasoning by analogy for us anyway :)