Sales breaks things out with a chart which would graph out the amount of "Wonder" a work has (how much does it fill up the imagination) and the "Agency" (is the work filled with possible things that work in ways they do in real life?), and I think of the scene in Dead Poets Society where the students are told to chart out the greatness of a poem immediately before Robin Williams informs the students that the idea is excrement and to rip out the pages of their textbook.
Sales goes on to say that
Works can, of course, straddle borders, which can lead to interesting effects. But as means of distinguishing between various genres, the above chart doesn’t rely on tropes – in fact, it completely ignores them. A story can, for example, feature dragons, defined as cryptozoologic reptiles, and be science fiction. A fantasy novel can feature spaceships which fly between worlds because some person in a cloak waves their hands and mutters gibberish.
I think his essay is worth a look. To the point that I have felt a need to truly categorize a novel or story as science fiction, I tend towards something Ray Bradbury once said about The Martian Chronicles (which I can't quite source),
First of all, I don't write science fiction. I've only done one science fiction book and that's Fahrenheit 451, based on reality. It was named so to represent the temperature at which paper ignites. Science fiction is a depiction of the real. Fantasy is a depiction of the unreal. So Martian Chronicles is not science fiction, it's fantasy. It couldn't happen, you see?
If I get technical about things, that's how I would define science fiction versus fantasy. It seems like, for the most part, Sales is working from a similar definition. He just has a graph.
But for the most part, I don't feel much need to define science fiction and fantasy. I can cover both of them under the umbrella of "speculative fiction" and I can further just say "genre" when referring to them. Of course, "genre" is non-specific and can lead to confusion with other genres, but then genre itself is fluid and allows speculative fiction to take some romance and take some hard boiled crime and mix it all together and you still have a speculative fiction story.
I can also say "hey, this is a really great book, you should read it" and recommend it like anything else.
Of course, Shaun Duke takes this in a different direction and calls SF a "Supergenre", but I'm not quite willing to dive into that at the moment. I just wanted to point it out.
My final thought is simply that science fiction (and all its subsets) and fantasy (and all of its subsets) are both under the umbrella of Speculative Fiction because they are different sides of the same coin. Of course, this would suggest that I should define "speculative fiction" in some way that is superior to all of the unsatisfying definitions of science fiction or fantasy unless I said that "Speculative Fiction is comprised of science fiction and fantasy and all the blurred lines in between."
I'm sure that doesn't work.