Look at these two statements:
"Much of our reasoning and so-called thinking is involuntary-- is as much a natural function as is the circulation of our blood..." CM Vol. 3 p. 115
"'Johnny can't reason.'" Said to William Zinsser by a college professor (Writing to Learn, p. 44)
I thought at first these were contradictions, but as I considered further, I decided perhaps we have so much trouble with reasoning ability nowadays because students aren't engaging with the content of their studies-- if they engage, the natural reasoning process Charlotte was talking about takes over, and as the students continue to commit their thoughts to paper, their reasoning ability increases.
One professor (in Writing to Learn) who had his students write essays on a values statement at the beginning of the year said, "Their papers were a disaster. They rambled all over." But as he encouraged them to write, and to present their ideas coherently and precisely, they improved. By the end of the year, "Their papers were clear and explicit. Their problems were in thinking, not in the mechanics of writing."
Writing, especially writing about what you are reading, really bumps you up against what you are thinking. There is something committal about putting your thoughts into visual form-- they are your thoughts, and there they are, in all their cock-eyed glory, reflecting the fog or the clarity of your mind.
"'Reading, writing and thinking are all integrated. An idea can have value in itself, but its usefulness diminishes to the extent that you can't articulate it to someone else.'" (Said to William Zinsser by a professor of history.)
"We think, and improve our Judgments, by committing our Thoughts to Paper." John Adams in a letter to his young son, John Quincy Adams.
"'Revising helps the students to rethink.'" Said to William Zinsser by a professor of Chemistry.
Watching someone do this can be fascinating.
"...the act of writing gives the teacher a window into the brain of his student. See Johnny reason! Watch him make a wrong turn! Follow his cogitations as he wonders what to do next!"