Hard and fast boundaries for genres like fantasy and SF, where their better members tend to be liminal, boundary-pushing stories anyway, are not going to happen. But it's easy enough to come up with a list of tendencies. Fantasy typically involves some kind of magic and takes place in the past or in someplace resembling the past. Science fiction typically involves some kind of science and takes place in the future or in someplace resembling the future. Any tendencies beyond those become less and less essential, except for those that the genres share: they both typically involve departures from reality, they both typically require both plot and characters (not true of all forms of literature), and so on.
Originally Posted By: Student of Trinity
I think the difference is this: science is bigger than any character in a book; magic is not. In sci-fi you can have background figures with transcendent powers; and among your developed characters, you can have badasses with better tech or knowledge than anyone else. But neither your heroes nor your villains nor even any of their super-scarey mentors are actually able to personally affect the laws of nature. Even if some character has a technological artifact with totally arbitrary powers, it will still be clear that the device is whatever it is regardless of the character's wishes. Even if the author hasn't spelled out what is possible and what is not at the beginning of the book, the texture of the story will be shaped by the fact that what is possible is in principle predefined, and not up for grabs to the characters.
Magic, on the other hand, means that what is possible is a lot more fluid, and in particular can be directly affected by the wishes of some of the characters. Whatever Tolkien might have said afterwards, within his story the impression is that in principle Gandalf might be able to do any particular thing he wanted. He is limited by his personal mental or spiritual 'strength', and by his ability to remember bits of lore; but I think any reader can tell, from the general feel of the story, that he does not just have a finite bag of particular tricks, and have to wait for a suitable opportunity for one of them. He can improvise reality, at least within limits, and his personal attributes as a character are on a par with at least some of the laws of nature. You could say that the character of Gandalf is himself at least a small aspect of the natural law of Middle Earth. This is magic, not science.
I have to strongly disagree with this theory. I think it is sometimes true, and I'll even wager to say more than 50% of the time, but not much more, especially not when you are talking about really original and lively books. Middle-Earth and Narnia, I would argue, are picture perfect definitions of worlds where what is possible is predefined. Tolkien's extensive development of his world, with its deep internal consistency, and his writings on this subject and on sub-creation make this explicit. Indeed one never gets the sense in Tolkien that the many coincidences in the books are a matter of random chance, but rather that things follow a certain pattern and arrangement:
Originally Posted By: The Hobbit
'Then the prophecies of the old songs have turned out to be true, after a fashion!' said Bilbo.
'Of course!' said Gandalf. 'And why should not they prove true? Surely you don't disbelieve the prophecies, because you had a hand in bringing them about yourself? You don't really suppose, do you, that all your adventures and escapes were managed by mere luck just for your sole benefit? You are a very fine person, Mr Baggins, and I am very fond of you; but you are only quite a little fellow in a wide world after all!'
The difference is just that the patterns concern what must inevitably happen with the system dynamics of the cosmos, from the gods down to the hobbits, rather than what must inevitably happen with the system dynamics of the cosmos, from the unified theories and galaxies down to electricity and robots. Narnia is the same way, when you look at what really goes on with the Deep Magic and Aslan and so on.
Not that I think either genre is all about determinism. There is a niche for free will within the Jobbish hurricane of system dynamics:
Originally Posted By: LOTR
The two powers strove in him. For a moment, perfectly balanced between their piercing points, he writhed, tormented. Suddenly he was aware of himself again. Frodo, neither the Voice nor the Eye: free to choose, and with one remaining instant in which to do so.