Of course, that might be exacerbated by her blindness/deafness, where for example a sighted person could probably conceptualize objects better without a specific language.

But is it true?

Are illiterate people incapable of rational thought? Oral traditions are deep wellsprings of human knowledge and culture.

Knowledge and culture is not the same as rational thought. Of course you can reason without writing, but the manipulation of thoughts is much easier when you can write them, the same as you can make calculations much more easily when you can write the operations.

You wouldn't know from the article, because the author of the article doesn't cite any studies and just sort of takes Neil Postman's word for it.

Writing things down doesn't just store and transmit information, it forces an author to crystallize and review their arguments. Writing certainly makes me more rational.

Those societies that never developed writing did have oral traditions with lots of "human knowledge and culture" but neither of these things are rational thought. Most oral traditions are rich with culture, but shot through with extreme irrationalism, exaggeration, fabuluism. This applies as much to a tribal society as to the stories your friends tell at the bar.

How many truly illiterate people do you know?

I suppose you could say that most 5 year olds are illiterate... are they capable of rational thought, perhaps... are they wellsprings of human knowledge and culture, I personally doubt.

In a literate society like ours, we tend to equate language with writing, but they're not at all the same thing. I agree that language is necessary for complex thought, just not writing.

For example, think of the first time you read a comment on HN which you impulsively disagreed with. If you gave it a second and a third chance, sometimes you come away with an alternate viewpoint, and sometimes you actually agree with the statement. That is the power of writing.

Now imagine what happened the last time you verbally disagreed with someone. Chances are, you soon branded them an idiot, and then slowly stopped talking to them. I would say a part of it was simply because the message was conveyed orally, with all kinds of meaning ascribed to intonations. And unlike reading a piece of text again carefully, its not as if you have a mental tape recorder you can use to replay the conversation. There is just too much distraction in oral communication.

To these ends I agree that oral traditions wouldn't "scale up" quite as well as written ones - but we're talking about pretty small groups of people, so they're going to have less viewpoints on the whole and therefore less need of a system that helps them manage intellectual pluralism. And as a side note, I'd think you don't really need multiple viewpoints to engage in deductive reasoning, if we're counting that as critical reasoning.

great orators with really bad messages could easily sway people down all kinds of wrong directions

Massive lapses of critical reasoning also affect post-literate Western societies.

think of the first time you read a comment on HN which you impulsively disagreed with...

You can impulsively disagree with someone and subsequently change your mind without the need to write everything they said down. You probably do this every day in casual conversation - especially if you're a software engineer - I'd bet most programmers are no strangers to impulsive disagreement. So I think your blessing here is really self-reflection - not a writing system. I think self-reflection falls under critical reasoning.

Now imagine what happened the last time you verbally disagreed with someone...

When you say that oral discourse has "all kinds of meaning ascribed to intonations", you're making it sound like speech is sort of a richer, more complex style of interchange than something purely textual like an email. Like, a speech act can have all kinds of richness and ambiguity. So wouldn't it then require more critical reasoning to parse out a speech than an email?