"One who can do anything..."

You thought, as a boy, that a mage is one who can do anything. So I thought, once. So did we all. And the truth is that as a man’s real power grows and his knowledge widens, ever the way he can follow grows narrowed; until at last he chooses nothing, but does only and wholly what he must do...

A Wizard of Earthsea, by Ursula Le Guin