"I began to develop my first law as a way to include magic systems that don’t follow very strict rules, but which also don’t undermine their plots. Let me state my law again: An author’s ability to solve conflict with magic is directly proportional to how well the reader understands said magic.
"This leaves room for those who want to preserve the sense of wonder in their books. I see a continuum, or a scale, measuring how authors use their magic. On one side of the continuum, we have books where the magic is included in order to establish a sense of wonder and give the setting a fantastical feel. Books that focus on this use of magic tend to want to indicate that men are a small, small part of the eternal and mystical workings of the universe. This gives the reader a sense of tension as they’re never certain what dangers—or wonders—the characters will encounter. Indeed, the characters themselves never truly know what can happen and what can’t.
"I call this a “Soft Magic” system, and it has a long, established tradition in fantasy. I would argue that Tolkien himself is on this side of the continuum. In his books, you rarely understand the capabilities of Wizards and their ilk. You, instead, spend your time identifying with the hobbits, who feel that they’ve been thrown into something much larger, and more dangerous, than themselves. By holding back laws and rules of magic, Tolkien makes us feel that this world is vast, and that there are unimaginable powers surging and moving beyond our sight.
"However, there is something you have to understand about writing on this side of the continuum. The really good writers of soft magic systems very, very rarely use their magic to solve problems in their books. Magic creates problems, then people solve those problems on their own without much magic. (George R. R. Martin’s “A Song of Fire and Ice” uses this paradigm quite effectively.)
"There is a reason that Gandalf doesn’t just fly Frodo to Mount Doom with magic, then let him drop the ring in. Narratively, that just doesn’t work with the magic system. We don’t know what it can do, and so if the reader uses it to solve a lot of problems, then the tension in the novel ends up feeling weak. The magic undermines the plot instead enhancing it. [...]
"On the other side of the continuum, we have hard magic. This is the side where the authors explicitly describes the rules of magic. [...] I would place Isaac Asimov on this side of the continuum. It’s a bit irregular of me to use a man who, from essays I’ve read, was generally disapproving of the fantasy genre. (Asimov argued that fantasy was about dumb people—men with swords—killing smart people in the form of wizards.)
"However, I think Isaac’s robot stories are a perfect example of a Hard Magic system. In his robot stories, Asimov outlines three distinct laws, then never adds any more and never violates those laws. From the interplay of those three laws, he gave us dozens of excellent stories and ideas. [...] You may be tempted to assume that superhero magic is a “Soft” magic system. After all, the powers are often ridiculous with reasons for existing that defy any kind of logic or science. (IE: “I got bit by a radioactive spider, then gained the powers of a spider!”)
"However, superhero systems are very much Hard Magic systems. Remember, we’re looking at this as writers, not as scientists. Narratively, superhero magic tends to be rather specific and explicit. (Depending on the story.) We generally know exactly which powers Spider-man has and what they do. He 1) Can Sense danger 2) has superhuman strength and endurance 3) Can shoot webs from his hands and 4) Can cling to walls. While in the comics, he does sometimes gain other strange powers (making the system softer), he does generally stick to these abilities in the movies.
"Therefore, we’re not surprised when Spider-man shoots a web in a bad guy’s face. We’ve established that he can do that, and it makes sense to us when he does it. It is narratively a Hard Magic system, rather than a Soft Magic system. [...]
"Most writers are somewhere in the middle between these two extremes. A good example of what I consider to be near the center point would be Rowling’s Harry Potter books. Each of these books outlines various rules, laws, and ideas for the magic of the world. And, in that given book, those laws are rarely violated, and often they are important to the workings of the book’s climax. However, if you look at the setting as a whole, you don’t really ever understand the capabilities of magic. She adds new rules as she adds books, expanding the system, sometimes running into contradictions and conveniently forgetting abilities the characters had in previous novels. These lapses aren’t important to the story, and each single book is generally cohesive.
"I think she balances this rather well, actually. In specifics, her magic is hard. In the big picture, her magic is soft. That allows her to use magic as points of conflict resolution, yet maintain a strong sense of wonder in the novels."
(via +gwern branwen