I always assumed the rules of physics would still apply to magic — no creation, just manipulating energy into different forms.

（ things taken, things given, things called into being. he's never thought too closely about shifters and their mass displacement issue. how does a man his size turn into a bird that weighs three pounds? something something alternative dimension, something. but he knows the mass doesn't disappear into nothing or, in the case of his grandfather turning into a grizzly, coming out of nowhere. ）

But I'm totally open to being wrong. Magic in my world is a little more 'sleight of hand for an audience'.