As I've mentioned recently on Twitter, back at the World Fantasy Convention in Brighton I had a conversation with SF author Mitch Benn about the new movie Gravity. I'm not sure how we got onto the topic, but he asked the question of whether Gravity was science fiction or not.
Now, I haven't seen it myself, and I don't know if he had, so any discussion here will naturally be a little limited by that. But I think it's a worthwhile topic here, since we've gotten to a point in our culture where a lot of things we take for granted could be considered science fictional.
If I recall correctly, my initial position was that Gravity was, in fact, science fiction, while Mitch was arguing that it isn't. Just being set in space, he suggested, isn't enough to turn it into speculative fiction, and I've come around to that position - unless there's something else involved in the movie, then it's simply an adventure movie set in space, and is speculative only to the extent that all fiction is speculative.
It wouldn't always have been, of course. If it had come out in the 1930s or 1940s, it would have been purely speculative, simply because nobody had gone into space before. But by now, space travel is pretty commonplace, if not as universal as we would have expected back then. If you count Gravity as SF, then you would probably have to count Apollo 13 as SF, too, except for one thing: Apollo 13 really happened.
On the other hand, when I brought this up on Twitter, somebody suggested we were getting into Margaret Atwood territory (ie, "I don't write sci-fi, because sci-fi is talking squids and lasers"). I don't think this is strictly true, because I'm not trying to distance myself from the genre, or to pigeonhole the movie into any specific box. But it's an interesting comment, because it plays up just how difficult SF is to define.
Some definitions suggest that it has to have an element of "prophetic extrapolation from the known"; others state that it requires a "human problem, with a human solution, which would not have happened at all without its scientific content".
The former works better for me, although I'm not 100% convinced by the term "prophetic", as something like Predator isn't actually prophetic, given that it takes place in the present day, but has an alien. The latter definition is much more problematic, because we come back to the problem of whether or not Apollo 13 would be SF; if you want to get really obnoxious, that second definition implies that, say, Jane Austen wrote SF, because even if unbeknownst to her, science underlies every experience she wrote about, down to the photons hitting Elizabeth Bennet's eyes when she gazes on Mr Darcy.
Sorry - I know that's taking things a little too far. To bring it back to a reasonable level, that second definition immediately turns all medical dramas into SF, which probably comes as a surprise to the creators of House. And, frankly, the Wikipedia page I just linked to contains so many definitions and explanations that cancel each other out, while also all being correct, that I'm tempted to just give up and go home.
As a final point, I'll just mention the suggestion that science fiction is a "mode", rather than a genre, as genre implies a certain amount of formula. This may be a problematic definition, too, as crime fiction encompasses a number of subgenres that each contain different tropes; but it shows that, as with Supreme Court Justice Potter Stewart's comment on obscenity in 1964, you can't define SF, but you know it when you see it.