How does that make a difference?
Uhm... how does it
not make a difference that when someone says "my brain hurts" that it's an idiom, and they do not literally mean that their brain hurts?
I might equally use "The doctors say my brain is in good condition." It makes no difference.
A doctor would not say that my brain is in a good condition, ordinarily, unless it was particularly significant--if there was some possibility that my brain was not in a good condition that was being investigated. Suppose that were the case--there was some reason to suspect I may have a brain tumor.
Then the doctor investigates a particular physical object--a brain--that I'm concerned about. Maybe this doctor investigates brains all day. But the particular brain I'm concerned about is in this particular head.
So how does the doctor say that the brain that
I am concerned about is in good condition?
The book's pages tell which pages. Your brain says which brain. And though I obtained factual knowledge that my brain is a very important part--even a
critical part--of my well being, I nevertheless have never developed a
sense of this brain. As such, I've never developed a
sense that I own this brain, akin to the
sense that I own my legs. As I said, it's not the same kind of thing.
"My brain hurts" suggests otherwise. But it's only an idiom--only a metaphor. Nobody has a sense of their brain. The closest you get is "my head hurts".