You might as well try to write a grammar book. What is a verb? Define it, please!
Back when people were living in caves, if Ugh the storyteller came up with a neat little tale about how those fluffy white things in the sky were sentient beings of some sort, and how when they darkened and poured down torrents of water it meant this thing or that thing, the tribe gave him a chunk of meat to eat even though he hadn’t been involved in the hunt.
That was a pretty good deal for Ugh. If he was a thoughtful person maybe he really believed his story was possible. Perhaps he was actually engaged in honest speculation. So let’s call him the prototypical science-fiction author.
On the other hand, maybe Ugh was a mystic who truly believed the clouds were departed spirits or whatever. So was he a fantasist, then? Well, if you allow the concept of "spirits" into your worldview in the first place, is it really mysticism at all? I would answer no. So Ugh should be considered a mainstream author in that case.
And in fact, since the estates of ancient cave dwelling authors aren't protected by current copyright law, you could legitimately riff on one of his old stories without fear of consequence: Cloud People—Today's Generation.
Ah, but you know that clouds aren't sentient beings, don't you? Then I'm sorry to tell you this, bub, but you've just moved into the fantasy authors' ghetto. The welcome wagon will be along to visit you shortly.
Now, at some point, some bright soul in Ugh's tribe probably noticed that water evaporates in the sun. And sooner or later people will generally notice that fog and mist feel wet, and that they both resemble clouds. So eventually the tribal consensus would have been that clouds are a form of water.
But as long as nobody could prove it, well, we'd still have some wiggle room.
Nevertheless, if Ugh's grandson had continued the "sentient cloud" series a generation later, when everybody finally "knew" that clouds were made of water, he'd probably also have been labeled a fantasy author.
Do you see what I'm getting at?
I recently saw a great article by Lois McMaster Bujold on this subject (it was on the CD-ROM that was included with Cryoburn). She opined that science fiction aficionados were seeking a "sense of wonder", whereas fantasy buffs were looking for a "sense of the numinous".
She's written some great stuff in both genres, so I'm inclined to listen to her.
Now, she didn't say this, but it seems to me that the "sense of wonder" people are basically in awe of the natural universe but don't care to have dogmatic explanations shoved down their throats. Which means their ranks will include atheists (who are quite the dogmatists themselves, ironically enough), agnostics (some of whom could go either way), and a large group of quite sensible people who intuitively have faith and are open to amazing possibilities.
Conversely, I think the "sense of the numinous" folks are more in awe of the very fact of existence itself, and are a little less blown away by its outward manifestations.. They wish to find something they're longing for rather than to discover something new.
What something? Again, their ranks will include many sorts of folks, and their answers to that question will vary all over the map.
There, is that all vague enough for you?
Obviously, the two fandoms aren't mutually exclusive. There are people who like both genres.
As far as that goes, lots of scientifically-minded people have a mystical/religious streak. I fall into that category myself. And I definitely like both genres.
How about you?