What is Magic?Edit
For such an integral part of the D&D world, magic, especially Arcane magic and Spell-like abilities, is scarcely defined in the rulebooks. Since magic and elementals are so tightly tied by convention, the place to talk about magic is here, in the book about elementals. The rules so far tell us a lot about how magic works, but nothing about what it is, why it works, or anything else of the sort. Most campaign settings make something up, although some leave it untouched, and because of this we can't give a definitive answer to this question. However, there are a number of answers as to what magic is not.
What Magic is NotEdit
This is far easier to define than what magic is, as long as one restricts oneself to ways magic has been written in source materials. Below are a few things that magic isn't but that nonethless certain DMs try to imagine it is and end up struggling against the game system.
Magic is not (the only) PhlebtoniumEdit
Most people, when asked what magic does for the D&D world, will say that it's what allows things that can't exist in the real world to exist. This is wrong. Zombies still walk, dragons still fly, and giants still don't collapse under their own weight in Antimagic Field, and none of them shows up on Detect Magic just for existing. Extraordinary abilities are perfectly fine with being physically impossible; a Barbarian can break through a stone wall with her fists as an extraordinary ability, for instance.
Then there are the people who play under Psionics are Different rules, where Psionics and Magic don't interact with eachother except through specific spells and powers, adding psionics as a separate type of phlebtonium. Then there are also a whole host of supernatural power sources that are just supernatural abilities. They don't show up on Detect Magic unless they come from magic items, but they are, nonetheless, supernatural.
None of this should be taken to mean that magic isn't the main source of phlebtonium in the setting. If you want something done, there's probably a spell that can make doing it much easier. But it's not necessarily the only way to do it.
Magic is not Free-willedEdit
Whenever you cast a spell, it does exactly the same thing each time you cast it, within your choices, die rolls, saving throws, and the like. You don't have to convince your Fireball to burn the orcs, you just cast it. The effects of the spell might be changed by your circumstances, but the spell itself has a single specific, scientifically replicable result. Unless your sword is itself intelligent or cursed or something, it doesn't just shut off without warning. Magic is not a free-willed force on its own, instead exactly obeying definite and predictable rules. That's not to say that there can't be free-willed intermediaries between you and the magic, they're just fine carrying on existing. Clerics have their gods, wielders of intelligent weapons have the weapon itself, and people petitioning an archmage to cast for them have the archmage.
Magic is not Constrained by Real-World PhysicsEdit
In the rules, there exist spells that actually permanently create matter from nothing. The spell comes and goes, and leaves a giant wall of stone, or iron, or whatever; you throw down your Feather Token and get a tree. The mass-energy equivalency of that oak tree in that feather token is over ten gigatons of TNT. That's far larger than any nuclear weapon ever tested. If magic were actually constrained to conservation of energy, destroying a feather token would be more than sufficient to destroy a small country, and the feather token would weigh the same in tree form as in token form. If the mass-energy equivalency were low enough to bring the energy in a tree feather token down to acceptable levels, then burning a piece of wood would take a substantial amount of its mass and get rid of it. It's also worth mentioning that the caloric requirements to cast a single Fireball are huge compared to the amount you actually might eat. If mages draw energy from within themselves and magic is actually constrained to physics, every mage must be ravenously hungry all the time. While you can design an interesting game on that principle, that's not D&D, or even NetHack; in D&D you can cast Create Food and Water and draw nourishment from it. I'm not going to go into the biology of that, especially where nutrients that are actually incorporated into your body from a limited-duration conjuration spell are concerned.
Then there are spells like Polymorph and Disintegrate. The former can set your mass without caring at all what your original mass was, and Disintegrate explicitly reduces it without doing anything with any kind of excess energy. You can change this if you make most of magic, essentially, into teleportation and summoning; change things so that a Wall of Stone isn't created, it's summoned. Certain D&D authors wrote it this way, even though it's not actually what their core rules say.
Magical effects aren't constrained by the amount of energy they use or by any other physical principle; otherwise Minor Creation would be a 9th level spell. Higher level effects are higher level for pretty much the metagame reason that they do more in the game, and some of them (I'm looking at you, Polar Ray) aren't even that; in the world, higher-level effects are just arbitrarily more difficult.
Magic is not ScarceEdit
Magic is not in danger of running out except on the smallest scale. The enemy sorcerer casting fireballs has no effect on yours; the battlefield is not in any danger of running out of fireballs. That's not to say that there aren't limits on how much magic can be crammed into a given body, or how much can pass through one in some amount of time; there totally are. They're called Spells per Day limitations. Magic can also leave lasting effects on places, which is similar to it running out, but instead of running out there's too much of it. More than a few Undead function in this way, including Bodaks and certain Ghosts, Necromantic Intelligences, and Forsaken Graveyards.
What Magic Might BeEdit
Since magic isn't so many different things, what actually is it? That's a tough question, and every game, or at least every setting, is going to have to answer it differently. Some of the possible answers are:
Magic might be one kind of energy in the universe, flowing around and being transformed into other kinds of energy as needed. This is fine, if you take a vague enough definition of "energy," and don't constrain yourself to real-world physics. Hence why this option is called "the Force;" you have to be talking about that level of divergence from real physics to make magic work in this system. Probably without the midichlorians.
Magic needn't be a single kind of energy, either; fire magic could be fundamentally different from acid magic, for instance, or Conjuration magic from Abjuration. Whatever you want to do. However, school of magic is given substantial preference in the D&D rules even though it doesn't resonate with anyone the way almost any other system of division does, with a few exceptions for schools such as Necromancy. Nonetheless, it's written deeply into the core dynamics of the game system, so school is the logical way to break down energy types.
In certain games, Magic is just a property of the world. The world is magic; you do certain actions and think certain thoughts, and the world responds in a certain way. This could be a sapient force, or it could just be a law of reality. This doesn't really explain magic, but it pushes the need for an explanation deep enough into the world that answering "because" will satisfy most players, and "you don't know" will delay most of the rest long enough. Since an explanation of magic need only satisfy the players, this is a perfectly viable option.
The Caster's WillEdit
Magic is often portrayed as being a manifestation of the will of its wielder. That works as an explanation, but doesn't explain the need for spell components, preparation, or even spells. Thankfully, this can be fixed by piling technobabble on top. Note that, for creatures that have spell-like abilities that they use at-will, this explains them well enough unless they are also mindless.
Standing on a TurtleEdit
In some games, magic invariably comes from an intermediary of some kind, whether a creature or object, physical or metaphysical. Forgotten Realms is the most obvious example, with their Weave. This doesn't actually solve the problem of what magic is, instead just putting an intermediary between you and the magic and pretending the problem's solved. If you ask how the intermediary does it, there's no answer. Enough people will be satisfied by putting something semi-comprehensible between them and magic to stop asking questions, though, so that solves that problem. Plus, this allows for progressively less-comprehensible turtles all the way down, which looks enough like a solution to work.
Where are the Limits to Magic?Edit
Because magic is so predominant in this world, the game world isn't complete without examining what its capabilities are. Hundreds upon hundreds of pages have been dedicated to defining the nitty-gritty details of what magic in D&D can do; indeed, an entire book has been published just to compile the chapters on this in every other book. This section will analyze, instead, what magic can't do. Anything else can presumably have a spell researched, or a strange phenomenon discovered, or whatever for it. While "magic" is most commonly referenced here, most of its effects apply to any form of phlebotinum.
A special shout-out has to be given to this spell, and its equivalent, naturally-occuring Dead Magic Zones. It causes huge problems when you have someone with a magic sword standing outside of one attacking someone with a magic shield standing in one, or trying to throw a magic weapon into one (or out of one), or even standing half in and half out (thanks to being larger than one square) and fighting in one - what happens to your items then? This spell will be rewritten to work more consistently in a later chapter.
There are nonmechanical issues here, too, where supernatural abilities include an entirely arbitrary selection of power sources, all of which are shut down, while another arbitrary selection are not. A high-level Barbarian really can laugh off a balista bolt to the chest in dead magic, but a monk can't put his fist through a brick in the same area. Class balance gets badly thrown off by these places, since classes with supernatural abilities expect to have them all the time (barring use limitations). Ultimately, the D&D world is one with magic in it, and taking it out entirely from some places doesn't make the game better.
Breaking the NarrativeEdit
Under no circumstances should magic, or phlebtonium of any kind, ever be allowed to break the narrative. That's not to say that magic shouldn't be allowed to derail the plot; that's the whole point of having it. If it can't, there's no reason to play a magic PC. However, around the table, causality generally runs in one direction. Events occurring around the table are reflected in the game world, and events occuring in the game world have to be talked about around the table. So causality in the game world has to have a definite flow to it; you can't affect the past since it's already been played through. The game quickly becomes impossible to play if the ability to travel back in time is thrown in casually; if you have to consider the possibility of a time traveller to show up and try to change things whenver you do anything, you have an adjucation nightmare. A few forms of time travel are acceptable; into the future (with no return) is fine, as is actually rewinding (like the Wish spell does); there's no risk of a time traveller showing up, and narrative time still flows in one direction, it just skips over things.
Everything that has to do with time in the game world should take into account the narrative time; causality only flows in one direction at the table, and the game world should reflect the way the table works. Trying to mess with the direction of causality in-world is fine in a single-author novel where you know how the (from an outside perspective) future is going to go, but doing so breaks down a cooperative game, not only through breaking your ability to understand how your actions will be reflected, but also through breaking people's understanding of how the world works and taking them out of the story.
Likewise, the fourth wall should remain intact. Magic should not "only target characters played by people wearing blue shirts", for instance. That just turns your game nonsensical.
Breaking the WorldEdit
Magic in high-end D&D is allowed to get away with surprisingly much compared to magic in pretty much any other source that tries to integrate its magic into the world. You have people habitually coming back from the dead, travelling bodily into the afterlife, creating worlds, and parting seas, all of which people seldom manage to get away with outside of myth. Still, once it's decided what the world is going to be, magic, and indeed all phlebtonium, must be restricted so that the world actually works like its supposed to. If that, for instance, means a world where armies are still viable, that probably means E6 D&D or some other way of getting high-level characters out of the way (or making armies viable against them). If that means a world where the gods or the afterlife are unknowable, then their domains cannot be accessible to Plane Shift.
Breaking the GameEdit
Naturally, some semblance of game balance must be preserved. D&D is established as a world where a Wizard can choose to turn into a nerfed version of another class as a hobby, so that kind of thing is actually perfectly fine at D&D's balance point. No, issues come in when people find exploits using magic to get around metagame constraints on their character, such as EXP totals. For example, the Thought Bottle's central involvement in so many theoretical optimization builds should indicate how broken its ability (simple saving and loading of EXP totals) is. Likewise, there should not be an infinitely-reusable form of magic to raise your level. It seems obvious, but it isn't. Things that cause people to underperform are also bad; if level loss from negative levels couldn't be restored, for instance, that would be terrible for the game because it would leave characters unbalanced within the party.
A special note has to be made of the Astral Projection spell, which bears mention because it's so obviously exploitable. It allows you to do anything you could normally do, on any plane of existance, at no personal risk barring a few specific dangers of the Astral Plane, like storms and Githyanki attacks (note that all Githyanki, barring their lich queen, are lower level than you if you can cast Astral Projection). But since, by the time you can cast it, you don't have to spend more than a single combat round on the astral plane, and can be somewhere safe to do it, that danger is trivial. If the D&D authors had honestly meant for top-end characters to only be in any danger at all ever (or have any items you can take from them) if they were being attacked in their sanctum, where they're hiding in a lead box behind 40' thick stone, that would be fine. But they didn't, since it's only a few classes that can cast Astral Projection.
So to fix it, your projection can only use six magic items, not eight, and is three levels lower than you. Your projection also doesn't have any items that you have that it isn't actively using, but it can use any disposable items on your person, teleporting them to itself as a free action. So now you don't get a free Scroll of Wish with every casting of Astral Projection, and any equipment created with the projection that the projection loses dissipates after one minute. Also, you count as having used every ability your astral copy uses, so you don't get your spells reloaded when you're sent back to your real body. A more complete rewrite of the Astral Projection spell will be given in the Book of Stars.
It's good for a cooperative storytelling game if everyone involved has some degree of ability to predict the results of their actions. Indeed, you can't really choose a course of action if you don't have any options where you know what will happen. So the effects of any given kind of magic have to be predictable. That doesn't mean that they have to be simple, just predictable. This also doesn't mean that it has to work the same way every time, just that when it doesn't, you have an explanation as to why. You could write a system where every effect was entirely simple, predictable, and self contained, but then you'd have Fourth Edition and that's not the way we're going. Indeed, it's better for the world if you have a system complex enough to get strong emergence out of your spells, because that creates a better illusion that the world's magic is actually something real. It's also better for the game, since it adds a degree of tactical depth to it. Not just simple tactics, like lighting Webs on fire, but also more advanced interactions, like using Mage Armor to encase yourself in force that crosses into the Ethereal plane to block off a passage.
Schools of MagicEdit
Since the earliest systemizations of D&D magic, it has been divided into schools based loosely on its effects. Those might have made sense way back in a wargame, with a clear line between effects that add pieces to the board (Conjuration), effects that damage pieces on the board (Evocation), and effects that protect your units (Abjuration), and to distinguish the Necromancer (bad guy magician who gets everything) from everyone else. But since then, very few of the schools have any real definition of what they do, or traction. Just Enchantment alone includes making someone do what you say by disincentivising everything else (Geas), making someone do what you say by overriding their will (Suggestion, Dominate Monster), and making someone like you by overriding their will Charm Person), but it doesn't include making someone afraid of you by overriding their will. That's Necromancy, oddly enough (although a Suggestion to run away in terror does work).
The schools of magic don't even have much traction. Really, aside from Necromancers, who are really just bad guy spellcasters who do bad guy things and cast bad guy spells while wearing black and skulls, and maybe Illusionists, which are a classic D&D trope, and NPC oracles (Diviners), there isn't a whole lot of traction for these kinds of divisions of powers. PC Diviners basically exist because they're the cheapest kind of specialist, and specialization is awesome, not because they want to play anything that might be stereotypically called a "Diviner." A Fire Mage is a more interesting character with more traction than an Abjurer.
While most schools of magic are distinction without difference, they are so heavily integrated into the rules that extracting them would be more trouble than it's worth. It would be nice to have a system where Detect Magic sliced up the spells along some other lines. Maybe it would tell you the difference between a fire or cold source of magic before it tells you whether said fire or cold is conjured or evoked. Maybe you won't even know whether it was conjured or evoked ever. But doing that would require coming up with a new system of tags (since there are a lot of spells with nothing but their school), and then assigning new tags to every single spell and magic item ever published, and that's a lot of work. So Wizard specializations and Detect Magic give information that most people don't care about, and the game goes on. Detect Magic can go on giving obscure information, and the game doesn't actually care if you phase out the Wizard and replace it with dedicated specialists.
Thermoarcana: The Entropy of a Closed MultiverseEdit
In the real world, if nothing else gets it first, the universe is eventually going to reach a "heat death" where entropy is maximum, all energy is either evenly distributed over the entire universe or in one single black hole, and nothing interesting will ever happen again. Indeed, the conventional interpretation of thermodynamics says that time only can be said to have a forward and back if entropy is changing, so if entropy hits maximum time may as well stop right there. The heat death is the end of time, leaving an endless black void without even a dock for your time machine.
The physics of a D&D world don't have to be set up to do this. The heat death is caused by the combination of the First (energy moves around, but doesn't come from or go to nothing) and Second (entropy always increases) Laws of Thermodynamics. These laws, though, don't have to hold consistently in the D&D universe. Messing with the Second Law is probably not a good idea unless you're careful, although you can suspend it in specific cases, like every time you cast a spell, if you need to. The only reason you see eggs shatter but not spontaneously fix themselves is the Second Law of Thermodynamics, but, in D&D and with the right magic, they fix themselves. Messing with the First works a lot better, and, indeed, much of D&D assumes that you do. Unless its being heated from somewhere, if the First Law holds, the Elemental Plane of Fire will eventually burn out even without the Second Law. Having the Elmental Plane of Fire need an outside heat source is dumb, and I'm not going to dignify it by writing for it. But, how this interplay works gives you three options for your multiverse.
Regardless of the option you pick, remember that thermodynamics wasn't really worked out until the 19th century in the real world. While steam engines probably exist somewhere in the D&D multiverse, which means someone will have to invent some semblance of thermodynamics to explain them, it's equally likely that most of the universe will be in the dark unless there's some obvious effect here. Also, it's hard to tell the difference between a system where entropy is decreasing and a system that is taking in some input that you haven't taken into account, an effect that to this day gives creationists trouble.
Thermodynamic Option 1: An Entropy ProblemEdit
The simplest option is that, like in the real world, entropy is increasing and things will eventually run down. In this option, the Plane of Fire is eventually going to burn out once it's leaked enough; the hottest hells will be cooled and the coldest hells will be warmed, the material will be consumed by the void, and so on. But that's all unimaginable trillions upon trillions of years away, so you don't need to worry about it. Under this system, though, everything that everyone and everything does increases the entropy of the universe. This probably means that conservation of energy (the first law of Thermo) holds too, or at least that the rate of energy creation is slowing down. So when you cast a Fireball, that fire has to come from somewhere. Not only that, but in order to have entropy increase, that fire has to be hotter than your Fireball; gathering up anything at all, be it matter or heat, and concentrating it reduces entropy (ignoring black holes for now).
Thermodynamic Option 2: Steady-StateEdit
If the net entropy of the cosmos doesn't actually change, then you can actually have it keep going the same way forever. Doing this, though, means that the indirect effects of magic have to be much greater and more significant than under the previous system. After all, entropy increases every time anything happens at all, so to keep entropy constant, entropy has to be magically reversed somewhere in the cosmos all the time. The universe might allow a slight "entropy debt" to accrue, but if the entropy debt gets too big it has to get magically reversed. This reversal could just be an area on some plane where things order themselves, or it could be something like "every time an egg breaks, someone, somewhere, casts Mending." This could even be as extreme as the Mending spell causing something to break somewhere in the multiverse, and heat engines working because a heat difference is being created somewhere by a Wall of Ice. Bear in mind that if you do that, you're basically saying that everything is caused by magic; messing around with fundamental laws of physics invariably makes things weird. If you play this up, this can work really well with Perfection in Balance morality to create a universe where everything is in unbreakable balance. Of course, unbreakable balance limits your ability to tell stories that involve, say, destroying large parts of the world. So there's a tradeoff.
Thermodynamic Option 3: A Growing CosmosEdit
It's also possible for the multiverse to be growing. The Plane of Fire can just be a literally endless well of energy. Dealing with that excess energy, though, requires putting it somewhere. Either it needs an endless sink to put it in, like the Plane of Ice, or it needs to have new space to put it in. The former leads to a semi steady-state universe, except without the wackiness caused by changing the second law, and with the ability to throw the balance off completely; you can, for instance, burn the world, without something else having to happen elsewhere to balance it out. The latter means that the amount of available space is growing. The planes themselves might be bigger tomorrow than yesterday, or there could just be more of them, but that energy has to be going somewhere. This also means that a wall of fire needn't actually be connected to anything at all; it can just make fire.
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