On a twitter thread, I proposed the idea that the main distinction between Science Fiction and fantasy is whether people prefer spandex uniforms or robes:
I did mean this in a tongue-in-cheek way. Obviously the only difference between Science Fiction and Fantasy is not the wardrobe. It is curiously harder to define than one would first suspect, though.
Before proceeding, I’d like to make a note that genres are not, or at least are not best considered as, normative things which dictate which books should be. Rather, they are descriptions of books for the sake of potential readers. The purpose of a genre is “if you like books that have X in it, you might like this book”. (The normative aspect comes primarily from the idea of not deceiving readers, but that runs into problems.)
Science Fiction is often described as extrapolating the present. The problem is that this is simply not true in almost all cases. It is very rare for Science Fiction to include only technology which is known to be workable within the laws of nature which we currently know. This is doable, and from what I’ve heard The Martian does an excellent job of this. At least by reputation, the only thing it projects into the future which is not presently known to be possible is funding. This is highly atypical, though.
The most obvious example is faster-than-light travel. This utterly breaks the laws of nature as we know them. Any Science Fiction story with faster-than-light travel is as realistic a projection of the future as is one in which people discover magic and the typical mode of transportation is flying unicorns.
I have seen attempts to characterize science fiction based on quantitative measures of how much of the science is fictional. This fails in general because fantasy typically requires only the addition of one extra energy field (a “mana” field, if you will) to presently known physics. And except for stories in which time travel is possible, the addition of a mana field is far more compatible with what we know of the laws of nature than faster-than-light travel is.
Now, one possibility (which I dislike) is that Science Fiction is inherently atheistic fantasy. This take, which I am not committed to, is that Science Fiction is fantasy without the numinous. Probably an alternative is Science Fiction is fantasy where there is no limit to the power which any random human being can acquire.
What I think might be the better distinction between Science Fiction and Fantasy is that Science Fiction is fantasy in which the author can convince the reader that the story is plausibly a possible future of the present. What matters is not whether, on strict examination, the possible future is actually possible. What matters is whether the reader doesn’t notice. And for a great many readers of Science Fiction, I suspect that they don’t want to notice.
In many ways, the work of a Science Fiction writer might be like that of an illusionist: to fool someone who wants to be fooled.
This puts Star Wars in a very curious place, I should note, since Star Wars is very explicitly not a possible future. But Star Wars has always been very dubiously Science Fiction. Yes, people who like Science Fiction often like Star Wars, but this doesn’t really run the other way. People who like Star Wars are not not highly likely to like other(?) science fiction. I personally know plenty of people who like space wizards with fire swords who do not, as a rule, read Science Fiction.
Anyway, even this is a tentative distinction between the two genres. It’s not an easy thing to get a handle on because it’s impossible to know hundreds of thousands of readers to identify the commonalities between their preferences. Even the classification of books into genres by publishers and books stores are only guesses as to what will get people to buy books, made by fallible people.