But when the wizard is onstage as the main character, you have to adopt what I call the Jack Vance Rule. I call it this because Jack Vance is the first author successfully and adroitly to have applied this rule in his The Dying Earth. The Jack Vance Rule is: (1) The wizard has to be able to do something unusual, or else he is not a wizard, (2) he cannot do everything, or else there is no drama; therefore (3) the story teller has to communicate to the reader whatever the dividing line is that separates what the wizard can do from what he cannot do, so that the reader can have a reasonable expectation of knowing what the wizard can and cannot do.
John C. Wright, Transhuman and Subhuman: Essays on Science Fiction and Awful Truth
fantasy, literature, wizards