You are going to love this theory. I guarantee it.
I was reading Andrew's article the other day, and that got me thinking about what makes magic work on a fundamental level in the cosmere. This will be somewhat of a rebuttal to you, Andrew, because this direction is far more fruitful. But it's much more than a rebuttal. In my opinion, this is quite a paradigm shift.
In any given Shardworld, magic is directly tied to a Shard. Everyone has pieces of Shards inside them. In Mistborn, everyone has fragments of Ruin and Preservation inside them. In Warbreaker, everyone has Breath. It's not unimaginable to think that this generalizes for any Shardworld, so I'm going to assume that always holds. In order to use Allomancy, you need to have "enough" Preservation in you. There's other stuff you need, too, like Snapping, but we'll discuss that later; it's not important anyways. The only real requirement for Allomancy is that you have enough Preservation. The rest is just book-keeping. And likewise, for Awakening, you need Breath, a fragment of Endowment.
So here's a question: why is Awakening different from Allomancy? Before you go saying this is a totally obvious question, let me remind you that there are distinct differences between Awakening and Allomancy. Awakeners never need to Snap, though Allomancers do (and the Shaod is sort of similar to Snapping, too, in a way). We want to discover the fundamental rules of how magic operates, right? So the question about the difference between Awakening and Allomancy isn't so dumb.
Now that I've made the question sound important, let me completely trivialize it with the easy answer: they are different because the Shards are different. Duh. That's fairly obvious. But in fact, this is precisely the key to understanding all magic.
My principle is simple: To interact with the spiritual power of the Shard inside you, it must be in accordance with the Shard's intent. (That is, the name of the Shard. Ruin. Preservation. Endowment. It is what the Shard wants to do. Look at the bottom here for that reference.) I'm going to call it the Principle of Intent, simply because that makes it sound important. Now I simply have to show you how important this principle is.
Let's say that Breath is the Nalthis equivalent of those fragments of Ruin and Preservation on Scadrial. These latent powers of a Shard can be accessed and manipulated. With Preservation's magic, I can do lots of cool things, but one thing I can't do is move that fragment of the soul around. But, I could do such a thing if I was using the right magic system. On Nalthis, Endowment endows. It makes perfect sense that Awakening can move pieces of souls around, because that's Endowment's intent. Indeed, I can do similar things with Hemalurgy, because it is inherently destructive. Ruin ruins. Endowment endows. Preservation preserves.
"But wait, why does Allomancy cause such rapid changes if it's from Preservation? That seems antithetical to Preservation's intent." There was a thread about this on Adonalsium.net, actually, and I couldn't come up with a proper explanation. Now I can. Watch closely, because this is fancy footwork. Think, for a moment, less about the external effects of Allomancy. Brandon's said that powers don't need to have rational explanations, just that they are bound by rules. Is there any particular reason why iron does the Physical Internal power rather than pewter? No, and that's okay.
Instead, let's consider what happens to that piece of Preservation inside you when you utilize Allomancy. Awakening moves spiritual energy, or endows it to something else. This doesn't happen with Preservation. There ought to be some reason why Preservation provides a net gain of energy with Allomancy instead of Awakening's system, and there indeed is. Preservation does preserve, it just preserves the piece of Preservation inside you (and for that matter, every aspect of your body. There is no direct physical cost to Allomancy in the act of burning metals). When you think about it from a Shard level, each Shard has a focus. Ruin and Preservation share a focus in metal. This metal activates some spiritual action, I'd imagine. Lock and key. The metal is the key to all the Metallic Arts. The metal must do something with respect to Preservation, and what's it going to do? With the Principle of Intent, that fragment of Preservation inside you wants to preserve itself, its spiritual energy. It also wants to preserve yourself. So, the only place it receive energy from is some external source, in order to preserve your own energy.
Slight side note: Another issue with Allomancy and its net gain is the idea that if Preservation "designed" Allomancy--though that term is a bit misleading, as I suspect no Shard explicitly could design something like that--he's losing energy. That's not true. The Well of Ascension refills every thousand years (technically, 1024 years). Utilizing it doesn't make it go away. Burning atium also regenerates eventually in the Pits of Hathsin. Likewise, there's no reason why this Allomantic energy actually goes away. It regenerates, too.
Wait a minute. I just got the most brilliant idea ever as I wrote that. Brandon said that Preservation or Ruin could, in principle, fuel any of the Metallic Arts, but he said that it expends power in ways gods are hesitant to do (I can't find the citation right now, but I think it's in the Hero of Ages spoiler thread). Why would it expend energy? Doesn't it all regenerate? No, I think not. Not exclusively. What if a Shard's power only regenerates when the Shard is doing something according to the Shard's intent? Atium is of Ruin, and so using that energy with respect to Ruining something will make it regenerate. The Well put Rashek into severe Preservation mode, making him extremely reactionary. So in a roundabout way, that again is Preservation. That kind of explains why dispersing the power broke Ruin's prison--it was an act not of Preservation, so the power wasn't conserved. Also, if this was true, it makes a heck of a lot more sense why Endowment Returns people, and why he isn't dead from doing so. The power all comes back eventually.
Ahem, that got off track. I'll build that theory in more detail, with better citations, later. Back to the Principle of Intent, with respect to the other Metallic Arts. It's been noted in the annotations that Ruin is selfish. So, fragments of its power wouldn't want to destroy itself, but it is perfectly okay with destroying other things. Hence Hemalurgy doesn't do much to yourself, and instead destroys other things. To use the analogy with Preservation, Preservation preserves spiritual energy, while Ruin destroys spiritual energy. I suspect that the greater amount of Ruin inside you, the more likely to perform Hemalurgy, however.
So why does Feruchemy work? Well, it's perfect balance. I can't utilize the power of either Ruin or Preservation here. They are balanced. I can't use either power up now. Were I to use or "access" the Shard's powers, they'd cancel each other out, so to speak. Nothing happens on the spiritual side of things (Okay, that's probably not true. It's just no spiritual energy is changed. I'm sure on some Realmatic level, both Ruin and Preservation act equally) So what happens if not doing something with spiritual energy? I draw from myself. This fits the Principle of Intent just fine.
You may be wondering why there'd be such a principle at all. Sazed could do all sorts of things when he held both powers. The difference here is that the body acts as a conduit--and not a very good one--for the power. Only once Vin's body vanished did the power become more expansive. I suspect this effect is due to the Physical, Cognitive, and Spiritual Realms. When a Shard's power is in a physical form, it does one specific thing (lerasium, atium). The Physical Realm seems to have a restricting effect. Thus, for humans, they access the power in their Shards in a more restrictive manner simply by being human. That's why the Principle of Intent appears to hold. Of course, even as a full Shardholder, the same principle applies eventually. The Shardholder's mind would be shaped to the Shard's intent over the centuries. Furthermore, Brandon said that Preservation or Ruin could, in principle fuel any of Metallic Arts, but he said that that expends power in ways gods are hesitant to do. Presumably, this is because while a Shard is a part of the power of creation and can do lots of amazing things, the Shard can more easily do things in line with its intent. Or, if I was right about the conservation thing, it's that a Shard knows the power will come back eventually. A Shardholder's consciousness apparently "feeds" off the spiritual power of the Shard, so if that power is all gone, that person dies.
Maybe you're not buying my principle. Don't believe me? Well, no theory is good unless it has some predictive power, so let's do some theorizing on other magics. I'd like to work with Elantris, but I don't know either Aona or Skai's Shard name, so it's going to be challenging to know just what they do. But Stormlight Archive is the perfect testing ground for a theory. We know all the Rosharian Shard's names, and there's a bunch of magic systems that don't exactly make sense.
I began pondering what exactly would Honor--the Almighty's Shard--do. Knowing the answer to this would give an intuitive understanding to the mechanism of his magic. But Honor is hard to pin down. Ruin ruins... Honor honors stuff. That's, uh, super helpful.
But you know what's really honorable? Oaths. Promises. Bonds. Oathpact. Surgebinder. Peter said on TWG that a Surge is what people on Roshar call a force (I'd assume things like gravity). Surge, plus a bond with a spren? Surgebinders. Congratulations, you are now a magic user. Honor's power comes from oaths and bonds. Perhaps it creates a spiritual bond of sorts. That's what Honor does.
There was also the question of what happened to Kaladin at the end of Way of Kings. He spoke the Second Ideal of the Knights Radiant, then he burst with power. Seems to me that this near unequivocal evidence that Honor uses these oaths in a specifc, magical way.
Then there's good old Szeth. How is he a Surgebinder if he doesn't have a spren? I'm not totally sure, but perhaps his strong oaths as a Truthless provides the sufficient bond for Honor's magic to work. Maybe there does exist a spren. Either way, those oaths are intricately tied with Szeth's Radiant powers. So, I'm calling it right now: if Szeth breaks his oaths, he will lose his powers.
I read a little bit of Way of Kings today, and I had forgotten that Jasnah said that two orders of the Knights Radiant have natural Soulcasting abilities. Now, recall that in the 17th Shard interview with Brandon, he said that there are ten powers (it seems to be ten Surges is the technical term), and you make an order of the Knights Radiant by picking from two of those Surges. I never realized before, but that must mean Soulcasting is a type of Surgebinding. That's kind of a scary implication. We've seen, what, Windrunners and Soulcasters, and there are ten more powers? Plenty more magic systems to go around, clearly.
But a more detailed analysis of Stormlight Archive magics is for another day, I think. Other theories for other days.
Source: The Principle of Intent