VirgilCaine said:The source of my comment on it being "a rule" is from the book "How to Write Science Fiction and Fantasy" by Orson Scott Card. I guess he doesn't count, huh?
I think that Card doesn't mean that the physics for creating magic have to be layed out. (That would be sci-fi.) Just that the author must show the borders and restrictions within which magic must operate.
Personally, I think that Rowling does a great job of delineating her rules of magic, slipping them into the story without paragraphs of exposition. E.g.
i) Magic requires study and practice, a lot of it
ii) Some spells are harder to learn than others
iii) Magic requires natural talent -- you either have it or you don't.
iv) Magic requires a wand
etc.
These restrictions make for fairly generic fantasy and so it's easy not to notice them. No-one's cutting off their limbs in order to channel magic, say. But the rules are still there. And for Rowling's purposes the rules are ideal: it's not the main focus of the story, plus, in some ways, learning magic is a lot like learning at say, math, at school, something her target audience can relate to.
The rules are still somewhat open-ended -- some borders aren't fully defined. We don't know what Dumbledore is capable of in a duel, say. That's not a problem to me as a reader. I think it's also true of other successful fantasy fiction.