I imagine the lack of grammar has also to do with the fact he used a typewriter. Something of laziness involved?
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There's a thread going on about the King James bible elsewhere on this website. Could be useful linking that one to this one.
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He simply wasn't the first to write this way. In my readings, once I got used to the grammar, stopped questioning it and simply read the words that were there, I just heard them in my head or as though someone were telling them to me, exactly like every other book except automatically in his psychological dialect.
As I see it, McCarthy already stated the literary world would be forced to look at the Judge and Blood Meridian,1985?, as the peek of his powers. The earlier work is depraved, too, but not written like BM, i.e., American Inferno. The rest would be made for Hollywood and for money. He had his interview on Oprah, remember, when he alluded to his child. I'm certain that factored in.