Look guys, it’s simple: science Fiction must be set on a space ship just as fantasy must be kind of European medieval land with wizards and elves. Everyone knows that.
Just thought we should get that reply out of the way…
See also phlebotinum, unobtainium, handwavium, and black box.
This has been part of debate about Frankenstein: the scientific explanation is missing.
Ug. Won’t let me edit.
Other 19th century fiction also has missing or glossed over explanations, like the Time Machine or the Invisible Man.
Also, stories about Golems from Jewish mythology are considered the first “robot” stories, but the explanation is purely mystical rather than scientific.
And then, there’s fiction where the science is just plain wrong, like the combustion engines on spaceships in Asimov’s The Foundation Trilogy.
No, it’s Clan of the Cave Bear without the sex. 
But seriously… what could possibly make it science fiction? Alternate history, genre fiction, speculative fiction… but where is the science?
I’d also add that since we have probably been eating roots and tubers since the time we moved out of the rain forest and became woodland apes it’s some pretty damn backwards-ass tribe that’s forgotten this.
Is The Time Traveler’s Wife science fiction?
Yes, I think this dovetails quite nicely with my proposed definition #3: science drives the plot, and without the science, the plot wouldn’t work.
Therefore, a romantic comedy set in space would still be a romantic comedy, not sci fi.
I haven’t seen it, but would Idiocracy qualify? I can’t imagine that they’d have too much technology we don’t.
Another boundary-stretching example: The anime The Wings of Honnêamise. It’s set on a world like our own, and centers around that world’s first manned spaceflight. The technology is about the same as our own Mercury program, but it’s clearly not a Mercury documentary (the nation launching the mission, and the historical events surrounding it, are completely different). Take out the technology, and there’s no plot left, so it qualifies by that definition, but there’s nothing in the movie more advanced than the real world had at the time it was made.
This is what I look for in a good SF movie, or at least what I might consider closer to “speculative fiction”. A good comparison is Alien vs. Minority Report. Alien could take place in an unexplored cavern, or forgotten ruin in the jungle. Minority Report asks “If there was a means to predict an act of murder, how would that affect society and humanity?”
The science part doesn’t have to be plausible or remotely possible. The point is to change something about the world, or reality, and see what happens.
What if robots were so lifelike you can buy an artificial version of the child you are unable to conceive?
vs.
What if we went into the jungle and got attacked by an invisible monster? I guess the invisible part makes it SF, the question isn’t nearly as interesting.
Wells didn’t gloss over the explanation in The Invisible Man – he gave a perfectly reasonable one. Granted, I don’t buy that it would work, but it’s there.
For that matter, Jack London gave two different possible explanations for invisibility in his short story “The Shadow and the Flash”
And Ambrose Bierce gave yet another one in “The Damned Thing”
In fact, there are lots of explanations for invisibility in the 19th century literature. Few workable ones, but they at least tried.
No, it’s a Just-So story – and an especially implausible one, at that. Whatever one’s opinions may be on the qualities of mutton in comparison to pork, “more bland” would surely be the least defensible.
Perhaps, but that misses the point. The broader question is, if you have a story about the initial development of some technology we take for granted now, way back in pre-history, is that science fiction? It’s definitely speculative fiction, since we don’t and can’t know exactly how it happened. And it’s definitely not fantasy, since there’s nothing magical about stone-age technology. So either it’s science fiction, or there is some niche in speculative fiction that is neither science fiction nor fantasy.
Alternate history and horror fiction are two niches outside of fantasy and SF but within speculative fiction. The story you described could perhaps best be pigeon-holed as alternate history… or even just historical fiction. (Though IANALibrarian) 
Another common argument: Star Wars isn’t sci-fi because the opening credits say “a long long time ago in a galaxy far far away,” therefore the spaceships and lasers and such don’t count as inventions.
This gives creedence to the #1 and 2 arguments I showed in the original post. Since robots don’t exist today, any film with robots, even set in the past, is sci fi. Therefore, a story like Clan of the Cave Bear or a story about cavemen discovering fire wouldn’t count as sci fi.
However, this argument doesn’t work because the technology is always changing. A flying car was invented this year. That doesn’t invalidate any sci fi work that previously used a flying car. Also, 2001, the “gold standard” for sci fi, was made just before the International space stations. The international space stations would not take away 2001’s sci fi status.
Another argument that comes up repeatedly: if we are perfectly happy with lumping a wide variety of work into one huge category called “fiction,” why do we even need to categorize something as sci-fi or horror or thriller etc.?
All those you list are sci-fi. Frankenstein is just as fantastic as the Island Of Doctor Moreau or Jeckyl and Hyde.
And of course things can be in two genres. To Catch A Thief is a crime movie, romance, and comedy.
Young Frankenstein is sci-fi and comedy and parody and costume drama.
The separation of horror as a distinct genre is really pretty artificial. Most horror is fantasy, since the villain is usually supernatural. Alien would be an example of horror that is science fiction. And a lot of modern slasher flics, where the villain is just some normal human who happens to be completely insane and/or evil, aren’t any sort of speculative fiction at all.
Sure. And it gets muddier depending on the writer’s explanations. Lovecraft’s horror has “old gods” (fantasy?) who are actually aliens (SF?) but whose powers are completely unexplained / magical in flavour (back to fantasy?)
Yes, for example while the books written by Thomas Harris (Red Dragon, Silence of the Lambs, etc) are probably Horror, they are also Crime drama, and Thrillers (?)… but not SF or Fantasy (except that Dr Lecter might be said to have super-human powers…)
But despite this muddiness I’ve still seen Horror as a separate section in libraries and bookshops.