A mind trained with the written word is different from a mind without it. The organization of thought required for reading is very different from that in an oral environment. The differences come entirely from communicative form.
Oral communication is nearly always discursive. Even when someone gives a monologue, it is to an audience, which reacts (perhaps silently) and participates. But monologues are rare and nearly always have a particular social purpose: relating important cultural narratives, or persuading people or expounding to them from a position of authority (what the ancients called rhetoric). But discourse is more typical of oral communication.
Discourse is by its nature unstructured. When you speak with someone, the other person can disagree, change the subject, extend your thoughts in a new direction, or bring up something new. Discourse is extremely unlikely to follow a set of logical presuppositions and explore them all the way to their end. By its nature it jumps around, assembling different ideas from multiple people in a back-and-forth which may or may not represent a coherent whole.
None of this is bad. It is just the nature of having multiple minds in real-time communication with one another through the medium of linear speech. Valuable knowledge can be imparted and also discovered in this process. A single mind following a single set of logical presuppositions cannot arrive at complete knowledge. But oral communication is by nature unstructured.
Not so the written word. Writing forces communication to be continuous and follow some particular path. There is no interlocutor to correct, derail, or add to the argumentation. If discourse is by nature a hodge-podge, with different thoughts from different minds combining to make a gestalt, writing has the ability to unmask whether the thought itself, expressed in language, has internal coherence. The act of writing forces the writer to pay attention to this. The act of reading brings to the attention of the reader whether what is being said has structure and consistency. Literacy is an avenue to greater coherence and precision of thought.
Literacy changes the way people think, or rather it opens up a new manner of thinking. It doesn’t necessarily supplant the discursive oral communication (elite Ancient Greek society, existing on the bleeding edge of the novel technology of writing, considered both oral and written language, in their proper uses, to be learned forms of culture). However, literate cultures have different qualities from illiterate ones. This kind of research is inevitably controversial, but it appears to be the case that written languages (even when they are spoken) more frequently use conjunctions and have more types of conjunctions. Many languages around the world lack a word for ‘or’, not to speak of ‘however’, ‘nevertheless’, or ‘yet.’ You can get on just fine with no conjunctions, or with a smaller number of conjunctions, or just a single generic conjunction that means ‘mostly and.’ This should not be surprising. If language occurs mostly in a context of unstructured discourse, there is less need for lots of connectives that link one set of thoughts to the next (contrariwise, there is more need for discourse elements acknowledging and addressing the interlocutor!). The increased attention to internal coherence in writing seeps back into the oral language here it is in an unexpected way: a multiplication of conjunctions.
Complex mathematics do not arise in oral cultures. This is not to say oral cultures cannot do math — you can find oral cultures comfortable with surprisingly high multiplication baked into their number systems. However, no purely oral culture has developed algebra or complex geometry. This kind of lengthy, step-by-step algorithmic process is something our brains are not naturally very good at. We seem to require an external aid for structuring, in the form of writing, to jump-start higher mathematics. After people are taught step-by-step mathematical processes, they can become quite adept at doing (some limited amount of) math in their heads. It just seems to be true that to take that first step requires writing the mathematical formulae.
We have achieved in virtually the entire developed world an extremely high rate of basic literacy. With vanishingly few exceptions, everyone can read and write either their native language or the dominant language of their local state. However, there have been two technologies that have changed how literacy functions in society.
2017: West High Reading Interventionist Teacher’s Remarks to the School Board on Madison’s Disastrous Reading Results
Madison’s taxpayer supported K-12 school district, despite spending far more than most, has long tolerated disastrous reading results.
My Question to Wisconsin Governor Tony Evers on Teacher Mulligans and our Disastrous Reading Results
“An emphasis on adult employment”
Wisconsin Public Policy Forum Madison School District Report[PDF]
Booked, but can’t read (Madison): functional literacy, National citizenship and the new face of Dred Scott in the age of mass incarceration.