I kind of feel like I need to clarify my last post about Clarke's Law. Before I even start on that, I want to say that it is my firm belief that every writer ought to write whatever sort of story she feels the desire or need to write, and that there is no genre or sub-genre that is automatically bad or inferior to any other. By and large, it's not what kind of story you're writing that matters--it's the execution of it. I even think this is the case for ideas and subgenres that I don't personally like or that I have nitpicky philosophical issues with.
That said. Some ideas are so profoundly foolish that it takes an unusual amount of work--handwaving, scaffolding, whatever--to make them work. And one of those is the idea that, assuming we're in a universe where "magic" works, science and technology are something different in nature from "magic."
I'm talking, by the way, about this issue on a worldbuilding level, not at the level of the characters in the story (though the one does affect the other, to a greater or lesser degree).
There was a time when "magic" in our world meant "stuff that works but we don't know how" and/or "stuff that works because the gods cause it to." We've increased our store of information about how the world works--largely through science, which is essentially a way of finding things out about the world--and now "magic" means "stuff that breaks the rules of the universe" or "stuff that doesn't actually work." But only the second sort of magic--the kind that doesn't work--can exist. The first kind can't. Not just in our universe, but in any universe.
If something works that contradicts your current model of the universe, then your model of the universe has to change to account for that. Anything that works, works because the structure of the universe allows or demands that it work. Nothing works over and above, or outside, the laws of the universe. "Magic" that works isn't magic, it's just the way the world works, no more mystical than levers or inclined planes or wheels. And the way it works will have implications for the way other things in that universe work. The "rules" of the universe are not handed down from some Physics Authority--they're a description of the universe, and if that universe includes, say, the ability to make it rain by pouring water onto a stone and saying the right words, then that's part of the description of the universe. It can't break the rules, because the rules are a description of what's possible to begin with. If sympathetic magic is possible, that's got to be in any accurate description of the universe in question, or your "rules" are in error.
The upshot of this is, as I said yesterday, that sufficiently comprehensible magic is indistinguishable from technology. That is, in a universe where magic is possible, magic is susceptible to scientific investigation, and use as technology. And once a given "magic" has been investigated, say, to use marycatelli's example, willow bark tea for pain relief, and found to actually be reliable, it will no longer be considered "magic." It'll be aspirin you buy at the drugstore. Any "magic" that didn't prove actually effective will remain "magic"--a category filled with things that don't actually work or haven't been investigated yet. (But mostly things that don't actually work, because most of the "things that haven't been investigated yet" will be things that do work but the mechanism isn't fully understood--and hence categorized as "not magic.")
As I said, this poses a problem for fantasy--or more accurately, for a certain sort of secondary world fantasy. Because if magic works, it won't act like what we call "magic," it'll act like, you know, stuff that works--steam engines or aspirin. Even if your characters don't understand why or how it works, it will work, and it will work according to rules that investigation will be able to determine, and at some point in your fantasy culture's history it will become "technology." Because magic that works isn't magic.
It poses a problem--but not an insurmountable one. It's not an accident, I think, that so many secondary world fantasies take place in worlds where scientific investigation doesn't exist on the scale we're used to, where the fantasy culture's knowledge about the natural world is lacking in places that would reveal the "unmagicness" of magic.
Even then there's a problem--magic-users summoning power from nowhere imply a universe in which matter and energy can, in fact, be created or destroyed. But I think that's fairly rare in published work. Most writers that spring to mind at least require some sort of source or payment for such things, at least gesture at balancing that out. And in the end, of course, if it's pretty enough I don’t care much how the logic works.
But there's no way for a universe to allow "magic" and not allow "technology" or "science" or "machines."
The conversation I mentioned in the other post happened years ago on a message board full of unpublished writers. The person in question described the world as one pretty much just like our own only, for instance, our kind of airplanes or cars or --for some reason my brain is kicking up vacuum cleaners, but I can't swear that was actually part of the post, we're talking half a decade ago I read this--anyway, airplanes, cars, or vacuum cleaners powered by mechanical, technological means won't work, but magically powered cars or airplanes or vacuum cleaners would work just fine. Which, you know, not possible. Because any condition that would cause all car engines not to work would also cause the human body not to work--it could only be true in a universe so incredibly different from our present one that the rest of the premise (that humans or vehicles, let alone vacuum cleaners, might exist at all) couldn't possibly make sense.
I'm not trying to call that poster out. I don't remember their name, and I suspect the post itself has disappeared into the aether. It was at least five years ago, possibly more, and likely they've thought more about things, or given up writing. They may well have been quite young--it's impossible for me to know. Doubtless they are a kind and generous person and well-liked by their family and associates. The fact remains, the idea that a world might be pretty much like our own only "magic works instead of science" is extraordinarily foolish.
And that post, my reading it and thinking, "Wait a minute..." was the place where I first picked up Clarke's Law and turned it around, and realized that it was true backwards as well as forwards, and on further consideration realized that the implication of that was that there was no such thing as magic that worked and never could be, no matter what the rules of your alternate universe were. Which is why I free-associated it into the conversation rachel_swirsky and I were having about a secondary world fantasy with routinely available magic.
As I said, there are implications for how your characters think about what you're calling "magic." If you've got, say, a late European medieval or European renaissance sort of setting, where quite a lot of technologies are considered mundane tech and not mystical or magical, and you've got a school for wizards, where dozens of wizard candidates come to study and learn spells--you might want to reconsider whether anyone in that society considers what those wizards do to be "magic" at all.
I personally think that's something you have to consider. It's not, however, something I think you have to come to the same conclusion about as I would.
One of the things I've noticed in slush is that I have problems with submissions that appear not to have thought any of the issues through that the story raises, but instead just grabbed bits of this and that and pasted them onto this other standard plot. I'm much more likely to enjoy--and pass up to the actual editors--a story that doesn't go along with my personal ideas about plausible worlds or whatever, but that is well thought-out, carefully considered. Or in which apparent incoherence is a deliberately chosen strategy, used intelligently.
Or, you know, I'll go with it if it's gorgeous, too. But I've found that gorgeous is nearly always coupled with intelligent and carefully considered.
This is just my opinion. (I have some opinions, y'all may have noticed.) This is not a slam on secondary world fantasy--hell, I write the stuff--nor am I saying anyone should or shouldn't write any variety of fantasy they want, or that any sort of fantasy is better or worse than any other. I'm just saying, if you write this particular sort of secondary world fantasy, I personally think it's necessary to think this issue through and decide what you think about it and what you're going to do about your conclusion.