Any sufficiently advanced technology is indistinguishable from magic. -Arthur C. Clarke
At my writer's critique group last week, one reviewer asked of a fellow-novelist, "Your character used a lot of magic here. What does it cost your character?" Like any good writer, he had an answer -- an answer he assures us will be made clear in a later chapter.
There are two types of magic in fantasy genres: Mystical and Scientific.
Mystical Magic unapologetically has no explanation. As readers, we are asked to suspend disbelief for the sake of the story. Magic just is, and you shouldn't question it. Gandalf appears out of nowhere, The Force is present in all things, and Excalibur is stuck in a stone because that's just how it is.
Scientific magic has a system, rules which a caster must follow, and reasons (implied or express) for its existence. As readers, we are let in on these secrets (more or less), and the plot itself is driven or bound by an internal consistency. Vampires must drink blood, mages must recharge their mana supply, and water spells always beat fire.
Traditional fantasy exists in a self-contained world, so it's easier to believe mystical systems. It's set so far in the past, or in a completely alternate reality, that it we don't always need an explanation. Magic just is, and that's ok. The evil overlord is evil because he ... well... isn't it obvious? Needless to say, there are many examples of scientific magic in traditional fantasy, for instance The Sword of Truth series by Terry Goodkind, The Wheel of Time series by Robert Jordan, and His Dark Materials series by Philip Pullman. But we're just much more likely to see Mystical, especially in older works.
Contemporary magic systems are more likely to use scientific magic. In the context of technology (cars, cellphones, computers), modern characters are used to having their questions answered (like "Where is that blue light coming from?" and "How does this telepathy stuff work, anyway?") If the character doesn't know (or at least ask), the reader will not suspend disbelief when the demon is summoned or when the second law of thermodynamics is broken.
In order to make the reader believe that the character believes, the writer has to explain. Or at least give the sense that there are rules, and that those rules are being followed.
The explanations themselves can be scientific in nature (genetics, radiation, toxic chemicals, or untapped human powers) but not necessarily. The important thing is that magic follows some set of consistent rules, and that those rules match some kind of internal logic.
A good magic system must contain a few basic elements. The easiest, and most obvious, is power. Magic can bring about the improbable and impossible. Mystic magic operates this way, as well. We don't really need a complex set of rules for this to be true.
The next most important piece is the cost of magic. If magic were free, then why don't witches rule the world? Wizard duels would go on forever, with nary a clear victor. So magic has to cost something. By default, it is assumed magic costs energy, be that in the form of mana, MP, blood, or just the need to eat more than usual. But some systems get delightfully more creative. Sometimes overuse of magic leads to a karmatic cost. Or one of my favorites (and the one used by my fellow critique-group novelist in the example above) you pay for magic by slowly losing your sanity.
You also need a believable (and hopefully interesting) casting method. It's kind of boring if your mage walks around snapping his fingers to shoot fireballs or make kittens disappear. Isn't it more interesting when there is some kind of ritual, chant, or coveted magic item? Harry Potter wouldn't be nearly so entertaining without the silly magic words. I love innovative spellcasting methods. One of my favorites was from the Myst video game series, where spells had to be sketched in a book before they could be made real.
Limitations. Magic must have some limits. Again, we wouldn't have a plot if a vampire with a year's supply of blood, or a mage standing on the world's strongest ley line, could instantly take over the world. In some systems, spellcasters are bound to the class of magic they are trained in (Necromancers can't heal and druids can't raise the dead.) Fast zombies die easily, and while slow-zombies are impossible to kill.. at least they're slow. I especially love spellcasting characters who always need a certain material to work with. I can't help but think of the anime series, Read or Die, in which the protagonist can only cast spells that affect paper. It's really fun to see how she resolves problems with this limitation!
Weakness is the last requirement. Superman has his kryptonite, vampires have their sunlight and stakes, werewolves have their silver, and faeries have iron. Not only do mythological characters need weaknesses, but so does their magic. This is often a paper-rock-scissors pattern of white magic beats black, air beats water, brains beat brawn. In my urban fantasy system, faeries depend upon human credulity in order for their magic to work. In the face of willful disbelief, their power crumbles.
What are your favorite magic systems? What is your favorite magical weakness, limitation, cost, or casting method? I'd love to hear it!