Genres are categories and often very broad ones at that, so you’ll have a tough time justifying the existence of a genre that includes only one text. However, we still haven’t really figured out how to define a genre, let alone whether science fiction and fantasy should be considered distinct genres. To do this, we need to identify patterns among the objects that might belong to a single genre and consider why we have lumped them together.
Comparing Apples to … Well, Other Apples
Comparing objects to identify both the differences and the similarities between them is sometimes useful for discussing very different things — like apples and oranges — but it may be more useful for comparing similar items — like red delicious apples and royal gala apples. These two varieties of apple are not exactly the same as each other, but then no two individuals of one variety will be identical, either.
Somehow, we need to pick out the important differences to identify the different kinds of apples. Should we concern ourselves with the exterior colour, size, and shape of the fruit, or perhaps the texture or flavour of the flesh, or some combination of the above? Maybe there are other factors as well.
To carry this analogy back into the world of storytelling, how do we know when two books (or movies, television shows, etc.) belong to the same genre? We need to mentally put them side by side and compare them.
Clearly, for two stories to both belong to a single genre, certain things just have to be the same. You couldn’t have a genre full of stories that had nothing in common any more than you could have a drawer full of random knick knacks whose utter lack of unifying features formed the basis for declaring “oddments” a particular class of objects. Repeat after me: a catchall is not a category.
They’ve Got Some of the Same Stuff …
So, what kinds of stuff are we concerned with here? Let’s start with settings.
Can the setting of a story be unique to that story’s genre? Fantasy is often set in historical times, but it shares this type of setting with both non-fantasy historical fiction and non-fiction history. Even science fiction may take place in a historical setting (think Back to the Future). And, of course, stories across multiple genres, including the two under discussion, may take place in contemporary settings. In other words, there is nothing special about any current or past time or place on Earth such that it would be an appropriate setting for one and only one kind of story.
Future settings, however, are often reserved for science fiction, either because the plot depends on some kind of new technology and its influence on society, or because the story involves time travel through the use of some seriously crazy and currently nonexistent gadgetry. Note, however, that time travel itself is not the sole domain of science fiction. Diana Gabaldon’s Outlander series involves accidental temporal wanderings, but the mechanism here is mystical rather than technological, pushing it towards the realm of fantasy.
Additionally, purely fictional worlds or alternative universes are common to both science fiction and fantasy. Yes, it’s true that fantasy is often set in worlds resembling real historical places while science fiction generally takes place in contemporary or futuristic settings, but any story that involves a completely fictional location will generally lean towards one of these genres or fall into the dim and hazy twilight zone between them.
So, science fiction and fantasy can have settings that are generally not found in other genres, lending some credence to the notion that they are variations of a single genre.
… And Some Not-So-Same Stuff
Fantasy and science fiction may employ settings that are unique to one genre or that are shared between them. How about features? And by “features” I am loosely referring to objects and creatures that may appear within a story.
Science fiction does have one feature that belongs to it primarily, if not exclusively: scientific knowledge and technologies that (a) currently do not exist and (b) affect society in some critical way. In some cases, these technologies play a prominent role, as does the tardis that takes Doctor Who through time to save the universe; at other times, they occupy a smaller, supporting role, as do soma and the “feelies” that replace real human experience in Brave New World. Whatever the case, the moment you introduce fictionalized science, you have science fiction, even if other genres influence the story.
Fantasy is a trickier case. Science and technology rarely appear here; instead, magic, dragons, unicorns, and all manner of strange, supernatural beings predominate, but many of these are like the critters found in various mythologies. Dust off those Greek and Roman classics you read in high school and you’ll find strange beasties of all kinds, including Pegasus, Hydra, and harpies, not to mention a plethora of supernatural phenomena. Is fantasy then just mythology that no one believes?
I’m not prepared to go that far. Fantasy books are known from the start to be fiction, whereas a mythology — by which I mean a creation story — is or was believed to be true by some group who used it to form the basis of their society. However, I’m quite certain that no one really believes that Lord of the Rings tells the true history of the rise of humankind, but I could be wrong. (If I am wrong, please don’t correct me. I’m happy in my little bubble.)
There are clearly some similarities and differences between science fiction and fantasy, but we still don’t have enough information to decide how the two are related, or even if they are truly related at all.