Language divides human consciousness
(Thanks to) the reduction of language to text ... an "interior" consciousness has been forced outward and virtually destroyed. (Havelock, The Muse Learns to Write 50)
What an incredibly perceptive thought--the notion that writing forces us to pour our ideas onto the page. I'm not sure that I agree with the assumption that the process is destructive, though.
It's interesting how writers explore their thoughts by brainstorming, but the actual process takes place both in our minds and on the page or computer screen. We think, write down or type some ideas, and then by observing those ideas--the ones we have externalized--we find it easier to develop them further. We even use visual cues like the shapes and lines of a web structure or the labels and organized patterns of an outline to quite literally connect and group the externalized ideas.
The page or the computer screen acts as an extension of our conscious minds, but I would argue that this doesn't necessarily cause deterioration; at least, not irrevocable damage. Perhaps it does change the way we think--we become dependent upon this extension to take full advantage of our ability to process information--but it doesn't permanently alter our minds. I would liken it to a muscle that grows weak from lack of use thanks to the development of a special tool (an extension) that makes a physically stressful task easier to perform. We still perform the task. We just do it in a different way that requires less energy, and if the tool were taken away, we would train and eventually bulk up enough to perform the task without the tool again. When we write, we still organize our thoughts and process vast quantities of information; it just requires less memory. If we suddenly lost our ability to write, I doubt we would be utterly lost. It would take time to adapt, sure, but in time we would train our minds to perform mental tasks without the aid of the written word.