Once, before I gave up arguing with anyone, I was arguing with someone and I wrote, “Science fiction is just a form of fantasy with stricter (but slightly inconsistent) rules. Fantasy fiction takes place in a world which does not exist, operating on principles chosen by the author; science fiction takes place in a world which doesn’t exist but might, operating on the principles of science as we understand it (with some cheating allowed in the form of time travel, FTL drives, Amazing Mental Powers etc).”
They weren’t impressed, as far as I could tell. Anyway, now I’d alter those formulations a little:
Fantasy is a mode of storytelling where all or part of a story takes place in a world which permits events that are impossible.
Science fiction is a genre of fantasy in which impossible events require some sort of scientific account or rationalization.
Science fantasy is a genre of fantasy that uses elements from science and technology but in which some impossible events are explicitly not given an adequate scientific account or rationalization.
I’m using impossible in a broad yet specific sense here. I mean “impossible now.” It would be possible (however unlikely) for me to go out and rob a series of banks, buy an airline ticket to Rome and have some decent gelato for dessert tomorrow night. (In practical terms, it’s just not going to happen: my own decisions abrogate this version of the future. So any daydream about it is a fantasy to the extent that my own decisions put it out of realization.)
It is not possible, in our current state of scientific knowledge and technological ability, for me to simply teleport myself to Rome and have gelato tonight. Maybe it will be possible someday, but at the moment it’s not. So, in a story, this would fall within the range of fantasy, and whether it’s sf or a different genre of fantasy depends on the account given of the teleportation-method.
But suppose someone believed that teleportation (as a psychic power, or a top secret government project, or whatever) was possible now? Then for them, the story’s not fantasy or science fiction. If the storyteller and the audience don’t agree on the shape of reality (as is not uncommon), the ability for a story to be fantasy (or to be effectively told at all) may be compromised.
In general, it seems to me, we should look at the writer’s intentions (an unfashionable theoretical position these days). A competent writer will signal genre intent. (And may give conflicting signals: that’s a fair variation on the storytelling game, it seems to me.) But the reader’s experience counts, too, and much that was intended in, say, the 1930s as science fiction is read as science fantasy today. This may look like a contradiction, but it’s really not. All genre categories overlap with others and exclusive definitions probably aren’t possible (or desirable).
Anyway, that’s what I’d say nowadays if I said stuff like that nowadays.