If I had to point to one idea that divides most fantasy from most sf, it would be “Born to the Power.” In fantasy, there are at least two sets of physical laws. One for the special people (magicians, heroes) and one for everyone else. In sf the laws of the universe hold for everyone: anyone can pick up a gun and fire it, as opposed to a magic sword which may only activate for one person. [Let me point out right now that I don’t believe that this is the end-all, be-all, one-size-fits-all, Final Answer to the sf vs. fantasy genre split. It’s simply been a handy rule of thumb that I’ve found useful over the years. Certainly there’s a lot of slipstream and magic realist fiction for which this idea wouldn’t be particularly useful.]
Let me use a specific case. There was a side conversation in this thread at Torque Control about Perdido Street Station winning the Clarke award as sf. (Paging Richard Morgan!) I tended to read it as sf because the magic in its universe is technological. There are many species with many differing abilities (like aliens) and the ReMade are much like cyborgs, and the scientist Isaac is trying to learn about his universe and its laws, which seem to consistently apply to everyone there. Even the appearance of the Slakemoth, which is by far the most fantastic element of the story, is much like an alien intrusion from another dimension (a hoary sf trope). Just because it’s not our universe doesn’t mean that Bas Lag isn’t a scientific universe. There’s no one in the story wielding unique power by virtue of birth or something similar.
I should mention that in ‘wielding unique power’ I mean some sort of magical or physical power. Even in our mundane universe, lots of people wield political power by heredity, and there’s nothing inherently fantastic about that. In fact, I believe that the “Born to the Power” idea stems from the historical notion of the divine right of kings: some people are simply more special that others, by birth, and nothing can ever change that. Thus we get heroes such as Aragorn--Gondor doesn’t get to vote about who to lead them, the story simply assumes (and confirms) that Aragorn is the right person, by virtue of birth. Whereas sf is literature born during and after the enlightenment, and (mostly) rejects many of those notions of inherent ‘specialness’ and looks at science and technology as a somewhat more level playing field.
This does lead me to a few odd categorizations: Star Wars becomes a fantasy because of the Jedi (and no silly ‘midichlorian’ ret-conning will change that). Gail Carriger’s recent Parasol Protectorate series becomes sf (as well as being steampunk and romance) because in its universe: a) there is a soul and it is measurable; b) anyone with enough of it can become a werewolf/vampire/ghost with some reliability; c) its existence or lack thereof appears to be heritable.
So I’m not drawing the line based on whether things are possible or not. After all, huge amounts of stuff in sf is not actually possible (warp drives, time travel, etc.) And I’m not assuming that sf has to be set in ‘our’ universe, or anything recognizable as such. I’m focusing instead on the physical laws of the universe of the story. Are they consistent? Do they apply equally to everyone? To go back to LotR for a moment, the dagger Sting is fundamentally technological--it glows in the presence of goblins no matter who is wielding it. But spell-casting magic is the preserve of the select few. By the way, having admitted that some of the magic in Tolkein acts as technology, I must admit that in my rule-of-thumb take on things, one drop of fantasy makes it fantasy. A story with a universe exactly like our own (or even with spaceships) except for one magic sword that can only be activated by one special guy winds up in the fantasy category.
One counter argument is all the super-spiffy heroes in sf: don’t they count as being ‘Born to the Power?’ Lazarus Long is an amazingly Competent Man, as well as being effectively immortal. That’s true, and it certainly indulges the same emotional satisfaction of having a super-special fantasy hero. But Lazarus was the product of a very specific breeding program, and while he is the longest lived of his brethren, he is not fundamentally unique in his universe. So I’ll still call that one sf. I don't need much of an explanation--I just need something I can pretend is an explanation.
I now look forward to teh Internets letting me know just how wrong I am. Thanks!