So that if you discover an elegant mathematical proof of something, you should make note of it in a textbook, but never get around to actually writing it out?
No thanks. I don't write in my books. The Prof can flunk me.
I also don't dog-ear the pages. I've discovered they all have numbers on them.
If I had one of Blake's printed books I would not write in it. But, any other book is to be highlighted, underlined, and annotated. I don't dog-ear the pages, but cut thumb indexes directly to vital passages--there aren't all that many in most books.