Proponents of the “magic in fiction has to be systematized” school of thought often defend their stance by arguing that without rules to circumscribe its operation, magic can do anything, and the story becomes meaningless.
I advocate for a slightly different framing of this idea: rather than focusing on magic having rules, think about its underlying logic.
Logic is less amenable to being laid out in terms that can be used to run a game, but that doesn’t mean it isn’t there. It just operates more in symbolic terms than numeric ones — and there are certain principles that commonly crop up in real-world belief.
Take the concept of “sympathetic magic,” commonly broken down into the law of similarity and the law of contagion. As described by Sir James George Frazer, they operate thusly:
First, that like produces like, or that an effect resembles its cause; and, second, that things which have once been in contact with each other continue to act on each other at a distance after the physical contact has been severed.
The “doctrine of signatures” is a specific example of similarity, developed in Roman-era Greece and subsequently bequeathed to European medicine. This idea held that if an herb resembled a certain part of the body, it could be used to treat ailments of that part. (Later Christian theologians explained this on the grounds that God had provided that resemblance so that humans would know of their virtues.) So, for example, walnuts were good for head-related problems, because their meat resembles a brain. Unfortunately for the real world, this doctrine is often dangerously wrong: Aristolochia clematitis came to be called “birthwort” because it looked somewhat like the birth canal, and was therefore used to treat issues with pregnancy. In truth, it’s toxic and can cause severe kidney damage.
But that’s only one example of how similarity can operate. In the U.S., where our paper money is largely green, it can mean using green things in rituals designed to bring wealth; where money is coinage, it can mean using one silver coin to attract more of its kind. Acting out the effect you hope to create is also similarity — thus putting visualization, sometimes employed with non-magical intent by people like competitive athletes, into this category. Similarity is highly flexible concept, and it can manifest in many different ways.
Contagion, on the other hand, leads to some interesting defensive behaviors. When people believe that their hair, fingernail parings, and bodily effluvia can be used against them in magic, they may take steps to dispose of those in ways that prevent others from obtaining them. (This is especially likely to be true for important figures such as monarchs.) One well-known method of using such things is commonly referred to in English as a “voodoo doll,” but since there are numerous problems with that term and the practice crops up in many cultures, it’s better to use the generalized word “poppet.” Contagion also doesn’t end with actual parts of the body: clothing and other items closely associated with a person can retain a link to their owner.
Calling these two ideas “laws” makes them sound like universal concepts, but in truth, they aren’t. A study of pre-modern Icelandic magic found no discernible pattern in the use of particular materials that might be sympathetically linked to the target; a magician could even draw a runestave with a fingertip on their palm and expect it to work. All that really mattered — our next principle — was intent.
Like sympathetic magic, intent is a widespread element of many magical beliefs. In Eden Royce’s novel Root Magic, which explores the author’s Gullah Geechee traditions, intent is specifically called out as key to the practice of rootwork. That element also underlies concepts like the evil eye: a single glare from someone believed to be a witch might be blamed as the cause of later sickness or crop failure.
But intent isn’t necessarily universal, either. Some concepts of witchcraft — and I believe this is true in parts of West Africa, but it’s unfortunately been too many years since I read about this to remember exactly where — hold that it’s possibly to unintentionally work witchcraft in your heart, through emotions like anger or jealousy. Those may seep out and metaphysically poison their target, whether you mean for them to do so or not.
One thing that definitely isn’t universal: the idea that there must be a defined source for the power used in magic. I suspect many fiction writers think in those terms because they’re consciously or unconsciously attached to the law of conservation of energy, or because they think it’s a necessary component to prevent their characters from doing magic too often or too easily. I won’t say this aspect is completely neglected in real-world magical belief, but it’s usually not approached in the same fashion. Much of the efficacy simply gets traced back to the divine; Icelandic magicians called (somewhat indiscriminately) on Norse gods, the Christian God, and demons to make their will into reality, but that’s not the same as conceiving of a divine power source being tapped into like a battery. God may have designed natural materials to advertise their beneficial effects, but those effects are inherent properties, not manifestations of an inner energy that gets drained by their use.
And even when there’s a clearly elaborated system of logic underlying magical belief, as with the Chinese five-element wuxing . . . not everything will fit into it. Academics have poured huge amounts of effort into trying to explain ideas like “turning your clothes inside-out will make you invisible to supernatural things,” looking for how that might be a specific manifestation of a more general principle seen elsewhere. And sometimes we have good theories! But some ideas might be born of one person’s idiosyncratic experience, or from a more logical practice that accidentally morphed in transmission. Or it might be a fragment surviving from a previous culture or imported from elsewhere. (The same is true of cultural practices generally, not just magical ones.)
So even if you’re thinking in terms of logic rather than rules, it’s possible — and realistic — to have random outlying bits that don’t slot neatly into place. Things that work for no easily discernible reason, simply because the world is like that. They can be the kind of thing characters wonder at or argue about . . . because after all, scientists working with real-world science don’t understand absolutely everything, either. It’s all more believable if it’s not quite tidy and crystal clear.