Sanderson's First Law

" An author's ability to solve conflict with magic is DIRECTLY PROPORTIONAL to how well the reader understands said magic."

Sanderson's First Law is a rule of fiction which claims that the better defined and understandable a magic system is, the more an author can use it in their plot. Simplified it can be interpreted as: "The better the reader understands a magic system, the more it can be used to resolve conflict."

This law was laid out and explained in an essay by Brandon Sanderson here on the subject of creating magic systems in fiction.

The Law is meant to express two things. First, if Magic A Is Magic A and it has rules, and the more rigidly and clearly these rules are defined and followed, the easier an author can incorporate the magic into a story without causing Fan Backlash or creating a sense of Deus ex Machina. The Law is advice to authors on creating and explaining the basic rules for their magic system in story, so that the readers understand what is going on and how/why it works. These rules can be like the ones used in The Dresden Files by Jim Butcher, where magic is very clearly laid out at a "nuts and bolts" level, with set limits and explanations on how it works in its most basic form and never exceeds the boundaries of what is stated. Or the rules can be more amorphous so long as the reader is able to understand just as well whether or not the magic is applicable to a given situation.

The second aspect is that poorly-explained magic should not be used to solve problems, only to create them. Then the protagonists have to solve the problem using more mundane means. This formula is good for building worlds that feel mysterious and dangerous, such as the worlds of Middle-Earth and Westeros.

Sanderson demonstrates the first aspect in his own magic systems, most notably in his Mistborn books where even when only half of the magic system had been explained, the readers where able to (correctly) determine the nature and abilities of the other half based on the information provided.