Brandon Sanderson recently posted a thoughtful article on the nature of magic systems, stating that the most important part of an author’s magic system isn’t in defining what things users of the system can do – it’s about specifying what they cannot do. If you haven’t read the essay yet, I highly recommend it.
In it, he users the mythology of Superman to illustrate his point – an example he chose to maximize the accessibility of his essay – but to bring the point all the way home for fantasy fans, I’d like to cite what I think is the best example: The Chronicles of Thomas Covenant, The Unbeliever, by Stephen R. Donaldson.
Without spoiling anything for those who haven’t read it, Covenant enters a magical world called The Land while wearing a white gold wedding ring. According to Donaldson’s magic system, the metal of this ring gives him access to immense power – he can actually unmake creation. Did you hear that? Unmake creation. Could there be any greater abillity?
The problem, for Covenant, is that he doesn’t believe. He doesn’t believe in The Land and he doesn’t believe in his power. He refuses to, because the simple act of even admitting to himself that he has that power – never mind acting on it – would weaken him back in the real world, possibly even kill him. In our world, his life depends on a total commitment to his own fallibility; his own impotence; his complete lacking of power. (It has to do with his medical condition, the frailty it brings him, and his need for eternal vigilance to keep himself safe from harm, but I don’t want to spoil the details for you.)
So we get a protagonist with enormous power who talks himself out of using it, despite all the atrocities being committed all around him, as evil forces threaten to destroy all that is beautiful in The Land. Covenant denies himself this power in order to preserve his own life back home and ends up trapped in a lose-lose scenario of his own making. In my view, the delicious agony of this premise is one of the most brilliant ideas ever explored in fantasy literature. And perhaps the greatest cruelty of this torture Donaldson perpetrates on Covenant is that Donaldson didn’t actually make this weakness a part of his magic system.
He gets Covenant to impose it upon himself.