What are the Elements?Edit
For all the attention D&D writers and players alike give to them, the Elements are remarkably ill-defined in terms of what they do for the world. Writers, DMs, and players come to the games with certain expectations about the elements, but they rarely write their expectations down and declare how the elements work, or even what each element means. Add that to the weird matching games done between elements and damage types, which aren't even played consistently, and nobody even has any idea what a "Water Monster" actually does. The end result is that the elements themselves end up a confusing mess.
This problem is compounded by the lack of real treatment of the way the Inner Planes actually function. There aren't any real answers in any publication if you want to find out what an elemental does when it's not on the end of someone's sword or ambushing you for mining their plane or running down a dungeon hallway to set off traps. Nor is it ever touched upon where they come from in any terms narrower than "that infinite space over there."
Elements Don't Work in PhilosophyEdit
The Elemental system used by D&D has a long history, with ancestors back to before the time of Socrates. There's a real possibility that people actually discuss that this elemental system is old enough that it could have spread from India to Greece before Socrates. Nonetheless, the D&D system isn't actually any of these systems, least of all Plato's interpretation of it. Fire is not shaped like a d4, Earth a d6, Air a d8, and Water is not a d20, and the stars are not made out of d12s. Really. D&D also has a long tradition of making new Psuedoelements in entirely incompatible ways and bringing in new elements from other systems (such as Wood from the Wuxing), that makes the entire Platonic system unworkable unless you want to go into epicycles on epicycles for it. With the two Energy planes, D&D doesn't even actually have a four-element system, but a six-element system, with Positive and Negative as its extra elements. At one time, D&D had to invent the words Paraelemental and Quasielemental, and give them different meanings, to add a whole 12 new psuedoelemental planes in, and they still brought in more. Really, every kind of Mephit once had its own home plane; they didn't all live on the major elemental planes. There were more kinds of Mephit back then, too. Then there are the Shadow Elementals and Genies in the Tome of Magic, making Shadow almost into an Element, which just brings it to where it should be with things made of shadow already existing.
Elements in classical philosophy also are the fundamentals of which everything else is made. If you use that definition of Element, D&D goes into crazytown right quick for its elements count, with almost as many as the real world (and few people try to assign complex and distinct symbolism to each of real-world chemical elements). Seriously, there are things made of Souls, Ectoplasm, Dreamstuff, magical Force, and so on that has no connection whatsoever to any of the Elements, even the expanded D&D set that includes Shadowstuff. But if the Elements aren't the basics of which everything else is made, what are they, and why make them special? They aren't even the fundamentals of inanimate objects, since there really are stones made of compressed souls; they're just one kind of the primal stuff of creation.
You can't even define Elements as "the power source of magic." Not to even get into the problems that has with making Elements into a mage thing, there are so many different sources of magic around that your number of elements would get ridiculous there. Is "Truenaming" an element under this system? Then there are all of the unique independent magical effects like, say, Color Spray. What kind of element is that?
The Elements aren't even consistent material. Real-world chemical elements are differentiated from eachother, but every atom of every chemical element is the same (except for mass and radioactivity) as every other atom of that element, and if you count isotopes as separate elements then even that goes away. The Elemental Planes are far from the undifferentiated mass that you'd expect to find if they were at all like chemical elements; the Elemental Plane of Earth is a mix of dirt, rock, and clay; the Elemental Plane of Fire even has giant plains of hot coals on it and a smoky sky, the Elemental Plane of Water has "seas" of different concentrations of various salts (and acids, and bases), temperatures, and so on, and the Elemental Plane of Air has clouds of every kind of gas imaginable, not to mention storms and dust.
Then there's the craziness where phlogiston filled the space between crystal spheres where Spelljamming ships travelled, and was completely distinct from the Elemental Plane of Fire. I'm not even going to try to justify that. They even got the theory backwards. See, phlogiston doesn't burn, and doesn't make things burn. Burning is when the phlogiston in something escapes. If the air's already full of phlogiston, escaping phlogiston can't go anywhere, so things can't burn. Chemists originally mistook nitrogen and carbon dioxide for phlogiston, because it was in air and stuff put in a nitrogen tank wouldn't burn, and oxygen was called dephlogisticated air because it could take phlogiston out of objects immersed in it by burning them. If you're flying through a bunch of phlogiston, things shouldn't burn, at all, and you might even suffocate.
All of these various interpretation issues about the elements arise because the concept has a lot of traction. There are campaign settings around that take the emphasis off of everything else that gets emphasized in D&D, but they still all pay some attention to the elements. Eberron makes bound elementals a cornerstone of its transportation infrastructure even as it redesigns everything else. Dark Sun makees clerics worship the elements since the gods are all dead. Players want elements; elementals touch something in D&D players' minds that nothing else quite touches, and there's never been actually a concerted effort to define what exactly the elements are; everyone has their own interpretation, and they've just piled up and stacked to the sky. So, as a result, the Elemental Planes are just there. They're environments that players can visit and mine, and they have their own empires in them, and that's all most writers can agree on.
Elements Don't Work in the GameEdit
The mechanics for the Elements aren't entirely consistent, either. Part of this has to do with the five major energy damage types, plus more minor damage types than can be easily counted, which don't match to the elements, like, at all, even in number. So Sonic damage usually gets left out, because it's actually something of a newcomer to the table of major damage types and was only invited because the game designers took pity on it. But you still don't have good matching. You end up with Fire matching to Fire most, but not all, of the time, Cold usually matching to Water, but sometimes to Air, Electricity usually matching to Air if it doesn't get Cold, Acid matching to Earth or Water, and if Earth doesn't get acid, then it gets something arbitrary, like physical or sonic damage. Then there are Genies' immunities, where Air gets Acid immunity and Earth gets Electricity immunity, just to go completely against the grain.
Elements are also usually treated as a mage thing, and that doesn't work. There aren't a whole lot of game mechanics that interact with the Elements unless you're casting spells or using magic items. For something that's important enough that the D&D writers invented four infinities dedicated to supplying enough of it to keep the world going, that makes us sad. Not only that, but even mages only have to deal with the elements if they want to. A mage who keeps an elemental theme is actually weaker than one who isn't, because she's not using all of her options. So, game-mechanically, the Elements don't actually affect anything except for specific classes and monsters.
The Elemental Planes are more important than the Elements themselves, but that's mostly because they're used as adventure sites. The infinite mass of Earth has actual gem seams and metal ores in it, and the DMG wholeheartedly recommends going to it to loot it for gems as an actual adventure. Once again, though, that's something for mages to do and the fighter can come along if the mage says so. The problem here, though, is that fighters don't get nice things; a fighter can't go to the elemental planes on their own without a Mage's help, but that's only because fighters are constrained to reality and mages to imagination. Fixing this by adding means of travel, from the Material to the Elemental Planes, like freestanding portals, fixes this problem nicely.
What to Do with the Elements?Edit
If it's so hard to define exactly what an element is, then what's to be done with them? That can't be definitively answered here; every gaming table is going to have to come up with that for itself. This question is actually a multi-dimensional question, since there's more than just the issue of what the elements are, but also what things are elements, and the answers to these questions interact in weird ways. But a few different answers, or combinations of answers, can be given, some of them better than others.
Theory 1: Just PlacesEdit
At one extreme is the possibility that the elemental planes are just places. Places that are rich in material and magic, but still just places. Campaigns that treat powerful magic casually work really well with this system. This system also makes the question "is this an element" irrelevent for any value of "this," since the elements, as a group, don't actually mean anything. This also makes answering questions about the cosmology fairly simple, since the reason things are the way they are can simply be "because."
Game groups that want to have deeper meaning for the world behind the other planes and for the things that happen on them would be less likely to want this option. If the planes are just places, like any other, if the Elemental Plane of Earth is just, basically, another Underdark, then it doesn't have the power it could have, if it were the source of all stone.
Theory 2: Multiversal FundamentalsEdit
Running to the other extreme is the possibility of making the elements into the fundamentals of the multiverse. This takes a very broad definition of the term element, or a lot of discipline in re-tooling the world's components to be able to pull off. If this path is taken, then it has to be decided whether Shadowstuff is an element or a kind of existing elements, and the same for wood. Positive and Negative energy have to be explained. What souls are made of has to be determined. It's a lot of work, but it brings a definition of the word "element" that's much closer to the way some people expect it to work than any other option. This option is good for those who want it that way, and bad for those who don't want to have to retool their setting. Although it's not strictly necessary to do so, games using this method may want to draw the elemental planes more to the forefront than they otherwise would.
Theory 3: Origins of CreationEdit
Something of a hybrid between the last two, it's fully possible for the elemental planes to be places like any other and to be the origins of the material world. Under this system, the elemental planes are possibly the oldest in existence, and definitely predate the material plane, which is made by mixing the elemental planes, either naturally or by deliberate effort, with the mortal world having been built up stone by stone by creator gods. When a choice has to be made to support one of these paths or another in this work's flavor text, this will be the one taken as default.
Theory 4: Planar Categorization SchemesEdit
Sometimes "element" or not might just be a categorization scheme for certain planes based on various shared properties. The inner planes have a bunch of things in common, such as their position in the planar arrangement, and the energy planes are clearly different from the elemental planes. The planes of ice and wood, though, are fairly similar to the proper elemental planes, so setting up a system that differentiates them out (especially one where ice and cold are out, but fire is in) is difficult.
It's even possible for whatever categorization scheme is used here to simply be wrong, just like calling these "elements" was historically. Maybe the categorizations of the elements were first done before anyone had ever been to the elemental planes, and were firmly entrenched by the time reports came back. That way, explorers would be out discovering planes and then trying to fit them into a context of a bad philosophical theory, which can explain any oddities about the way the elemental planes are set up. This is actually assumed to be true whenever people try to bring in something esoteric from real physics or chemistry into their D&D world. If the D&D setting has, say, Cesium in it, then the elemental planes don't actually correspond to real elements.
It's also possible for it to be correct, but based on something esoteric. The kinds of spells that show up on spell lists is one possibility, as are frequenly-seen cleric domains (although this breaks down with Plant if wood isn't supposed to be an element).
Theory 5: Physical CorrespondencesEdit
A popular idea is to draw correspondances between the elemental planes and four of the states of matter. Earth is solid, water is liquid, air is gaseous, and fire is plasma. That works as a system that matches the four elements to something scientifically real, if slightly mistaken (most fires aren't hot enough to create meaningful amounts of plasma), and also strictly defines which planes get to be in the elements club. However, it has the problem where magma, molten iron, and so on would be found lying about the Elemental Plane of Water, rather than on the Plane of Fire, where they might be put by those who don't fully buy into this model. Because of this, the parts of this work dealing with the Inner Planes are going to ignore this theory. Some affinities between elements and certain spheres would have to be redefined, too; for instance, Metal would be entirely an Earth sphere. The Energy Planes and Psuedoelemental planes don't fit in this scheme very well at all.
Elements and Transitive PlanesEdit
There was a major shift in the arrangement of transitive planes on the way into 3rd edition. Back in earlier editions, the Ethereal Plane connected the Material and Inner planes through the "Deep Ethereal," so magic to become ethereal worked on the Elemental planes. The Astral only connected the Material Plane to the Outer Planes. Shadow wasn't even a full plane, just a large demiplane in the Ethereal that nonetheless had a point corresponding to every point on the Material plane, just like the Ethereal did. It's a weird system, but it certainly worked, if not under the current definitions of how Demiplanes work.
The current arrangement has the Astral Plane connecting to everywhere, even the Inner planes, and the Ethereal plane only connecting to the Material Plane. Deep Ethereal still exists, and doesn't actually go anywhere except a few hidden demiplanes. That works, if you're willing to waste an infinity, so for the purposes of this writing the Inner Planes will have their own Ethereal Planes, which connect to the Deep Ethereal the same way the Material's does. This also means that the Inner Planes are actually Inner, since they're more closely connected to the Material Plane, and the Outer Planes are actually further away, since the Inner Planes have every connection they do and more.
There's also a new Deep Shadow for 3rd Edition, which connects to alternate Material Planes. This could be as much as other campaign settings, or it could be Evilverse or Bizzaro World, where your PC has an evil counterpart with a goatee and your nemesis's counterpart is a great hero, or a world where everyone except your evolved cat is the opposite sex as they are on the Material plane, or whatever. It's where you find the opposites' dimension, that's the important point. Now, you'll note that if the Shadow dimension has access to any of the same Inner/Outer planes at all, you don't even need to go through Shadow. Maybe there are Bizzaro Outer and Inner planes; maybe there aren't. Alternate material planes are basically a campaign-specific gimmick anyway.
The Trouble with InfinityEdit
Note that every plane mentioned here except for demiplanes is infinite in volume in the core rules, mostly as a cheap, lazy way to get around being expected to provide a map. Some of them, like Ethereal and Shadow are even bigger than the Material, which is itself infinite, since they have at least one Depth dimension in addition to all point correspondences (note that when travelling in a Transitive plane away from the border regions where it's coexistent with another plane, directions don't matter over any significant distance). If they have a non-zero population density (which you can probably assume is true of Deep Shadow), then they have an infinite population, too. When an infinity gets in your problem, or your solution, it invariably messes something up. While it might seem nice to have an infinite amount of empty space to dump garbage in, or hide your demiplanar castle in, you also have an infinite number of people producing adventurers. At the same time, large parts of the cosmology, say, the number of Deities, are finite. This simply will not do.
One obvious solution is to make everything infinite, but that also makes everything even more pointless than it already is with a few infinities; even deicide is useless if there's an infinite number waiting in the wings. Follow along for a moment, and try to visualize an infinite plane. Pick your favorite elemental plane. Set a density of settlement on it. Pick something you can comphehend, like "three days walk from one town to the next" or whatever. Now visualize the entire Milky Way Galaxy. Spread that cosmic background population density over the entire volume. You're looking at a diameter of about 58 million billion miles here. With even one settlement for every square a hundred miles on a side, and no vertical stacking, we're still talking more settlements than stars in the sky. Millions to Billions more. With even one adventurer per settlement, you have enough allies to make your contribution fade into the crowd, and enough enemies that you can't even guarantee that it won't be undone. Remember, that's all without even leaving your one plane of existence.
You're not done, though, after just one galaxy. Remember that there are entire galaxies far enough away that you can't see them with the naked eye. Fill out to that distance at your settlement density. You're still as far from infinity as you were when you started, but even this is too big to cross with anything short of Greater Teleport, and too vast for your character to have an impact anywhere but the tiniest scale on.
The problem can be reduced a bit by putting a strict beginning of time in, so that settlement has only expanded so far. This only works if you put it recently enough, though; an empire that has been growing, in fits, for sixty-five million years, or four billion years, or whatever might as well be infinite, but if it's made of immortals it's plausible that a civilization of only 6,000 years is still a concievable size. This almost even works, but the D&D world has creatures that spontaneously generate in it. It doesn't matter if the Genie Empires have only had six thousand years to spread, since elementals and mephits just pop out of nothingness, and they also produce the occasional hero-level threat. So even putting the beginning of time in recent history doesn't help.
So we get rid of the infinities and make Planes and Demiplanes no different, but true planes generally range from many times the size of the Earth to the size of the Solar System and up, and are usually much more densely packed with interesting things than the Solar System. This runs into problems with infinite age, so creation needs to have a date set for it, or at least a date set for the emergence of the last planar civilization, otherwise everything will be filled up because you've put an infinity in. Nobody needs to know that date, of course; just that there was a time before there were Genie Empires and a Blood War. You can stick unfound borders somewhere; Deep Transitive planes are good for this, since travel in them doesn't work like it does elsewhere. Then you even have a cosmology with room to explore so that you can keep it from getting filled up or completely revealed, which is like having infinite planes except without any expectation that the new places will be at all like the places you know and love.
The exact sizes of planes is left unspecified here, within the range of "larger than earth" and "as big as the Solar System." Walking from the sun to the closest orbital point of Pluto would take over three centuries at 24 miles a day, and would take half a decade if you teleported a thousand miles every day. So a plane the size of the solar system is genuinely as big as it needs to be for most adventures. It's still impossible to get around any significant part of except for with teleportation. A DM who wants to do things that way can easily manage with just finite size by setting the size at "astronomical."