If you’re writing a fantasy, chances are there’s some sort of magic or magical force present in your story. Not having a defined system for that magic is like playing a game and making up the rules as you go along. Everyone else will know you’re cheating.
I’m not suggesting you write out a list of rules and affix them to the endpapers of your novel like formulas in an algebra textbook. What matters is that your magic follows a logical system. You can allow readers to infer the rules over time through demonstration.
But why must there be a system? Doesn’t that defeat the awesomeness of magic?
No. Actually, it’s the other way around. Anything is possible when there are no rules, so even awesome abilities become arbitrary. Tension only exists in a story when there are foreseeable consequences to actions. These are the stakes. When you use magic as a dues ex machina to solve your problems on a whim, it feels like a cheat. It implies that none of the consequences were ever of concern, or perhaps worse, that the author is bending the rules of the world whenever it’s convenient for a certain character or the plot.
The eagles that repeatedly save Gandalf and the others in Lord of the Rings are a bit of a dues ex machina. They save the day when it’s convenient for the story, but aren’t available otherwise. Is there a reason Gandalf can’t just call the eagles to fly Frodo and the ring up to Mount Doom in the first place? That sure would have saved a lot of trouble. Because there’s no defined system for when and where the eagles can save the day, it feels like Tolkien could just write them in anywhere.
If Harry Potter had suddenly realized he could solve all the world’s problems by clapping his hands and clicking his heels, all his prior struggles would have been pointless. Part of the series’ success can be attributed to its clearly defined magic system. Rowling designed limits to what magic could do, and special procedures and even consequences for using it. Without this structure and rules, anything would have been possible, and the wizarding world would have lacked tension.
It is not what the hero can do that’s important, it’s what they can’t.
By limiting the magic in your world and defining a system, you enable the building of stakes through potential consequences. Readers can trust that the author is not going to ruin the tension by making up arbitrary conveniences as they go along.
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