Loosely defined, magic is anything which isn't possible in the universe we know. It often involves special powers and the ability to bend the laws of physics. In Fantasy literature magic is extremely common, and even alluring. Many readers turn to certain books, movies, or television programs because they enjoy pushing the limits of what is and isn't possible.
Magic makes anything possible.
The first question an author needs to address if using magic in their world is what are the limits and costs of magic? Does the magic in your world have restrictions? A character who can perform limitless magic, with no cost, is usually called a god. These characters have been popular in literature since Ancient times. Homer wrote about the gods, just as teens go to the movies to watch Thor. In other fantasy novels, some characters have different limits or different costs than other characters.
Limits are boundaries as to what magic can or cannot do. Since magic essentially replaces the laws of physics, the question that arises is are there other laws or rules which govern how magic works in your universe? Let's look at a few concrete examples of what magic can/cannot do in some examples of popular fiction.
The X-Men, pictured above, probably aren't the first group of fictional magicians who spring to mind, but as characters able to completely bend or alter the rules of physics, these figures all practice their own form of magic. They're also uniquely limited. Each mutant has one power, that only they can do. Wolverine cannot read minds, and Magneto doesn't control lightning. Stan Lee devised a world where the magic is limited by dividing it equally among characters. Everyone has one gift, and nothing else. This works well in confrontational situations, where powers clash. This is the basis for many superhero stories. Other authors who divide their magic to create limits include Piers Anthony's Xanth books, where each character has a single "Talent" and is unable to do any other type of magic. Michael DiMartino's Avatar the Last Airbender series is an example where entire nations possess a similar magical ability, and the different nations clash. A fire-bender controls fire, whereas a water-bender controls water and so on.
Another option, if limiting magic, is to choose things that magic simply cannot do. In Disney's version of Aladdin, the genie pops out of the lamp, filled with magical potential, and immediately tells Aladdin there are "rules". There are things he cannot do for Aladdin, which in that case include bringing back the dead, making people fall in love, or granting unlimited wishes. It's one of the clearest and most concise examples of a magician spelling out the limits of magic in his world. It's important to make sure your rules coincide with your plot. If Aladdin wishes for unlimited wishes, he becomes a god. If he makes Jasmine fall in love with him, the entire point of the movie ends. And of course, if he brings the dead to life, Disney probably won't make the movie. An alternate take on putting strict limits on what magic can do, is to put an internal limit, as to what the magician is willing to do. In Christopher Paolini's Eragon series, for example, the magician Ergaon has the ability to draw energy (which he needs for magic) from living things around him. He is unwilling to kill those things, which adds depth to the character, while also adding a slightly fluid barrier to the limit of his magic.
It is, as was mentioned, a viable option to have limitless magic. If an author goes this route they often put limits somewhere else. The most popular books about magic ever published introduces a world with nearly limitless magic.
The scene pictured above, from the film adaptation of Harry Potter and the Goblet of Fire, is one of the scenes where JK Rowling discusses the three "forbidden" curses in the Harry Potter world, namely the powers to torture, control against one's will, and murder. These three curses are limits, yet it's important to note that all three are used by characters in her books. In other words, the ability to use this type of magic not only exists, but is necessary. In Harry Potter, there is only one specific limit given to magic, which is that magic cannot bring the dead back to life. All other magic is possible, so Rowling employs laws which restrict the uses of magic. She's added limits to the characters, since the world is nearly limitless.
If an author creates a world where magic is limitless or nearly limitless, they often create limits elsewhere. One such mechanism is to increase the cost of magic.
Prince Allan stares at his four remaining fingers, trembling. He remembers the terrible pain, but he has no choice. He closes his eyes...
In the above example, which I just made up, a character is born with the ability to use magic whenever he wants. His magic is limitless, yet the cost is specific. Each time he casts a spell, he loses a finger. He can therefore only use magic ten times. Hopefully he uses his powers well.
In your world, it's important to know the cost of using magic. Often there is a penalty. To return to the Eragon example, in Alagaesia (Eragon's world), the cost is life energy. If Eragon casts a powerful spell that never ends, he will die. If he's weak he has to either draw energy from somewhere else, or wait until he's healed to cast the spell. In George R.R. Martin's Song of Ice and Fire the cost of potent magic is royal blood. This helps propel several plotlines, especially around the figure of Stannis. The magic used is literally blood magic, coming at a dire cost. In video games and anime, many worlds incorporate an actual monetary cost. If you have enough money, you buy a spell. The more money you have, the greater your power will be.
The cost of magic doesn't have to be external. In Tolkien's Lord of the Rings, Frodo offers Gandalf the One Ring very early in the story. Gandalf refuses, as do later characters, saying that if they used the Ring's power to overcome the villain Sauron, they would be turned to evil. The cost of using the Ring is to lose oneself. Similarly, in the Star Wars universe, Jedi have the ability to use the Dark Side of the Force. Yet doing so turns them evil, as is the case when Annekin becomes Darth Vader after using the Dark Side.
If magic doesn't come with a cost, can anyone use it? To return to Hogwarts and the Potter world, there is no cost to magic in Rowling's world. A student can spend all day at school, and if they know all the words, they can cast a thousand spells with no repercussions. To compensate for what would otherwise be a series about a god going to school to train with other gods, Rowling tells us that only certain people can use magic, people born as wizards. They must also use magical extensions, such as wands in order to facilitate their power. In other series, Rick Riordan's Percy Jackson, who is himself an actual demigod, is limited by his human emotions. At the end, when he is offered godhood, Jackson declines, in favor of a mortal love.
As a fantasy author, there's no right or wrong way to do magic. Magic can be limitless, or be so controlled that its nearly impossible to use. Magic can have no cost, or have a cost so high, you fear using it. What if the cost of magic was the life of your child? Would you use it? What if magic could do anything at all, except the one thing your protagonist needs?
Fantasy opens all of these possibilities and more.
The discussion of magic will continue next week...