Can't remember where I got this from: "The author starts with a skeleton of an argument and turns it into prose. Your job is to take the prose and excavate the skeleton."
If the excavated skeleton is messy, either a) you're a bad reader, or b) the author is a shit thinker.
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I actually remember this passage. But the context he's talking about is about story, and how he lets the story lead him (as opposed to plotting the story beforehand).
This quote is by another thinker, I think. And it's about reading more than it is about writing.