Intellectuals who propose that "someone who never wrote might still be the greatest thinker, but no one would ever know." fail to acknowledge, that they wouldn't be here except for ancient people who could 'think' like animals, --and were successful hunters...
i suspect (having all the grace of an elephant in a china shop) that if i had to hunt food to survive, i would be dead in six months (if not sooner!)
It seems evident that many illiterate people function very well in our society, (but how many of us have the skills to survive in a stone age society--or even a simple hunter gatherer society?
When life depends on the ability to make twine, and fashion the twine into an effective snare, or into a net to catch a fish, or into a foot strap so we can shimmy up a tree trunk to harvest fruits or nuts growning out of reach--which of us would survive.
We have reached a very comfortable point in society, we don't need basic survival skills, (and many of us don't have them!)
who is more clever? the first being who realized that plant matter could be split, and then piled, and then woven into a mesh to make a net (and successfully harvest fish) or any modern day intellect who spends hours parcing a paragraph?
We can easily pooh-pooh 'simple machines' like the wheel, the lever, the screw, the pulley. but these remain, wonderful technological acheivements, made by, (for the most part) societies that had no written languges. and i do think writing/reading plays an important part of 'thinking' in language.
I don't think the screw(as a tool) grew out of 'discussions' or focus groups, or 'teams' of designers with talking points and bullets. It came from thinking--and the person doing the thinking was thinking in images.
and i think we(US/UK/western european culture)often fail to acknowledge or value this sort of thinking.
but (to para phrase a quote) societies that value their philosophers more than their plumbers, will soon find, neither their ideas (or their pipes!) hold water.