If anyone hasn't heard of/read Brandon Sanderson's specific writings on the topic of the logic of magic systems, they are extremely relevant to this discussion. They've been summarized very well
on Wikipedia, which I quote below. I like that he has applied this to sci-fi as well, because his comments echo very nicely something that Asimov said about writing a successful sci-fi murder mystery - something Asimov was pretty good at.
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Sanderson's First Law is that "An author's ability to solve conflict satisfactorily with magic is directly proportional to how well the reader understands said magic." While originally created as a rule for magic systems in fantasy novels, Sanderson has specified that this law need not apply just to fantasy, but is also applicable to science fiction. This Law was originally defined in Sanderson's online essay "Sanderson's First Law". In the essay he qualifies the two extremes of design as being:
Magic/technology has well defined rules that the audience understands. As a result, one can use this to solve conflict more easily as the capabilities are cleanly defined. Sanderson classifies this as "Hard Magic". C.L. Wilson in her essay "Worldbuilding 101 - Making Magic" advocated this method of creation, stating, "...create your rules, then follow them."
Magic/technology has unclear or vague rules, or none at all. This allows for a greater sense of wonder to be attained for the reader, but the ability to solve problems without resorting to deus ex machina decreases. Sanderson classifies this as "Soft Magic". Lawrence Watt-Evans specifically advised "The trick is to be a benevolent and consistent deity, not one who pulls miracles out of a hat as needed"
Sanderson's Second Law is "Limitations > Powers", that a character's weaknesses are more interesting than his or her abilities. It was initially set down in Episode 14 of the podcast Writing Excuses. John Brown, likewise looked to Sanderson's work in his own essay involving magic systems, noting "What are the ramifications and conflicts of using it?" Patricia Wrede likewise noted several issues on this topic ranging from magic suppressing other technologies, to how a magic might affect farming. In explaining the second law, Sanderson references the magic system of Superman, claiming that Superman's powers are not what make him interesting, but his limits, specifically his vulnerability to kryptonite and the code of ethics he received from his parents.
Sanderson's Third Law is that a writer should "Expand what you already have before you add something new."
Sanderson's Last Law is that a good magic system should be interconnected with the world around it. Sanderson points out that magic does not take place in a vacuum. It is related to the ecology, religion, economics, warfare, and politics of the world it inhabits. The job of the author is to think farther than the reader about the ramifications of the magic system. If magic can turn mud into diamonds, that has an effect on the value of diamonds. Sanderson states that readers of genre fiction are interested not just in the magic system but how the world and characters will be different because of the magic.
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