I ran  across this extremely interesting and totally meta article on fantasy magic by Brandon Sanderson. I’ve had my own version of this kind of theory since the early 80s but he really spells it out.

He breaks magic systems down into how “hard” or “soft” they are. Meaning, how defined are their rules.

If you’re a writer working on your fantasy magic systems, I suggest that you decide what kind of feel you want for your magic. Do you like the techno-magic like you find in my books, or in books by L.E. Modesitt Jr. and Melanie Rawn? Do you like the hybrids like you find in someone more like David Eddings or J.K. Rowling? Or, do you prefer your magic to be more vague and mysterious, like you see in Tolkien or the George R. R. Martin books? I like to read works by all of these authors, but when I write, I prefer to have rules, costs, and laws to work with in my magic, and that makes it more fun for me.

By hard or techno-magic he means books like his own, or comic books, or video games (like WOW or Diablo), where the magic is a well defined tool. On the soft side are books like Tolkien with a more mythic feel. But what is particularly interesting is his insights into the narrative impact of magic.

Resist the urge to use magic to solve problems unless you’ve already explained and shown that aspect of how the magic works. Don’t give the heroes a new power whenever they need one, and be very careful about writing laws into your system just so that you can use them in a single particular situation. (This can make your magic seem flimsy and convenient, even if you HAVE outlined its abilities earlier.)

Very good advice. If your magic is soft enough that major new developments occur every time it is used, then you better NOT use it to solve problems. Or:

If you’re writing a soft magic system, ask yourself “How can they solve this without magic?” or even better, “How can using the magic to TRY to solve the problem here really just make things worse.” (An example of this: The fellowship relies on Gandalf to save them from the Balrog. Result: Gandalf is gone for the rest of that book.)

Semi-consciously, this is what I did in The Darkening Dream which has multiple complex magic systems that are not fully explained, and is hence a kind of middle-soft magic universe. The characters do use magic, but it rarely helps or pans out the way they want, and when it does, I’m generally using a power that I clearly set up before. The villains make heavier use of magic, and their systems are better defined. Still, things often go poorly. In this book I really wanted to give the magic a sense of weight. To make it clear that it was never free or easy and required years of study, practice, and consequences often far outweighing the long term benefits.

My second novel, Untimed, is in many ways closer to a hard magic system in that the time travel has very rigid rules. This (and the related villains) are the only “supernatural” element. I try to maintain my sense of mystery in a number of ways despite this increased definition. 1) The characters are young and alone and don’t know all the rules. 2) They find them out as they go and by trial and error (emphasis on the error). This is also useful to avoid bombarding the reader with too much infodump. 3) I keep the “how” this all came to be and the “why” the villains do what they do hidden throughout the first book.

To me, this balance of the feel of the mystical world is absolutely essential to fantasy writing. How much I like a story is heavily influenced by it, even if I am a fan of tales across the hard/soft spectrum. Like everything, execution is key.

Find Brandon Sanderson’s entire article here.

A detailed write up on the basis of the magic I used for The Darkening Dream is here.