The truth is that good fantasies carefully limit the magic that's possible. In fact, the magic has to be defined, at least in the author's mind, as a whole new set of natural laws that cannot be violated during the course of the story. That is, if at the beginning of the story you have established that your hero can make only three wishes, you better not have him come up with a fourth wish to save his neck right at the end. That's cheating, and your reader will be quite correct to throw your book across the room and carefully avoid anything you ever write in the future. All speculative fiction stories have to create a strange world and introduce the reader to it - but good fantasy must also establish a whole new set of natural laws, explain them right up front, and then faithfully abide by them throughout.