Recently in a comment thread the question of whether a particular concept was science fiction or fantasy came up, and I realized that I just don’t believe that to be a meaningful question any more.
I will try to explain why.
When I and the world were much younger, I used to play a tabletop role playing game called Champions, which was developed by Steve Jackson Games and contained many of the concepts that grew into GURPS (the Generic Universal Role Playing System.)
Because Champions was a superhero genre role playing game, and comic book superheroes have such a broad range of abilities, the designers didn’t try to codify every possible ability. Instead, they created a system for constructing powers that was based solely on game mechanics, and left it to the player to define them.
So if a character had, for example, a power that did energy damage at a distance (ranged energy blast) it didn’t make any difference in terms of game mechanics whether that was a blaster or a magic wand.
That’s a concept that I have gleefully adopted in my fiction. In terms of the mechanics of my story, it doesn’t matter if the Outsiders are alien minds or demonic presences. What is important is what they can and can’t do to my characters. By keeping them deliberately ambiguous, I hope to keep my readers from taking them for granted.
I am also indebted to C. S. Lewis’s Space Trilogy (Out Of The Silent Planet, Paralandra, This Hideous Strength) in which discusses the difference between science fiction and fantasy at some length.
Other writers who I feel share this sensibility are Whitley Streiber, who has written science fiction versions of werewolves (The Wolfen) and vampires (The Hunger), Scott Westerfeld who tackled vampires via parasitology (Peeps and The Last Days), Tim Powers, who… well, I don’t know if anyone knows exactly what Tim Powers does, but, damn, he’s good at it.
The (in my humble opinion) sadly underrated film Reign Of Fire is a post-apocalyptic science fiction film about a plague of dragons. (I suspect that the genre confusion may have contributed to its poor sales.)
The point is, I don’t believe that there is really an significant difference between science fiction and fantasy. There are specific locales and character types that together tend to define a particular type of world in the reader’s mind, but to my mind the primary difference between Spok and Legolas is costuming.
I am sure that there are people who have a strong opinion that there is a difference, and what that difference is, and I’d like to see your thoughts. I may have missed something significant here.