I really do wonder! You see, I'm the kind of person who instantly says, "Science Fiction" when you ask me what genre "incarceron" falls into. But my sister insists that book is fantasy, and I'd guess most of the world agrees with her, and not with me. (I LOVE "Incarceron" and "Sapphique, by the way. If you haven't read them yet, you really should.)
Then I was reading a recent book review of Ursula LeGuin''s stories, and some person (a man, I think) began slamming her for not writing "real" science fiction. Ursula LeGuin! One of the finest and most famous SF writers in the world! But she writes fantasy, too, of course, and her SF is concerned with people and societies and how they intersect. That her Hainish books deal with alien worlds and spaceships and cool tech like ansibles wasn't enough for the fellow who was criticizing her. As far as he was concerned, SF had to extrapolate what life would be like in the future, and had to do so based on hard science. I can't think of a single SF author I like (Fisher, Bradbury, Duane, LeGuin, Walter M. Miller, and several more) who actually writes science fiction defined that strictly.
I'd still say, though, that all these authors do write science fiction. They often write fantasy, too, and Fisher and LeGuin in particular often create worlds combining elements of both genres. But I think it's easy to tell when one of their books is primarily SF. Here's my definition:
If there is magic, it's fantasy. Otherwise, if it's concerned with the future, spaceships, time travel, aliens, or other SF tropes, and there is NO magic, it's SF. Simple! That's why I kept insisting that "Honor" is science fiction. Future setting, spaceships, no magic - it has to be SF, doesn't it?
Well - actually, looking at Catherine Fisher again, I'm not sure. I'd define both the Incarceron/Sapphique story and the Relic Master quartet as SF, certainly. But the ARC my sister loaned me of the first book in her new series? (It's called "The Obsidian Mirror", and, like everything she writes, it's really good.) It's got strong SF elements, certainly - time travel among them. But it also concerns faerie and seems to have magic - a genie in a bottle, the Winter King and the Summer queen, an enchanted wood . . . Finally, it has Shakespearean echoes all over the place. "Hamlet", "A Midsummer Night's Dream", and "The Tempest" are obvious influences. So - SF or fantasy? In this case, I'd say fantasy, in spite of the science fictional elements. But that's because the faeries introduce an element of magic.
I still think that, if you've written or are reading a speculative book, and there is no magic in it, it's SF. That's my definition, anyway! What do you think? How do you define SF, and how does it differ from fantasy?