The borders of science fiction (as a genre) have not always tracked the distinction we have made in this series. Consider, for example, C.S. Lewis’s Out of the Silent Planet, Perelandra, and That Hideous Strength. Written loosely as a trilogy, C.S. Lewis intended these works to be treated as science fiction. He wanted to create science fiction works that were imbued with Christian and theistic themes. He wrote, “I like the whole interplanetary ideas as a mythology and simply wished to conquer for my own (Christian) p[oin]t of view what has always hitherto been used by the opposite side” — that is, the side of naturalistic atheism or deism.

In these books, C.S. Lewis describes Earth as the “silent planet,” because its inhabitants no longer converse and consort with the divine beings that grace other planets in the Solar System, because of their wickedness and cruelty. Human beings are a fallen race, unaware of races that live without the sin that keeps Earth in the dark. In short, while having the aesthetic of science fiction, the in-story universe is morally inflected in a way that today would disqualify it from science fiction, from the framework we have offered here. C.S. Lewis wrote, “What immediately spurred me to write was Olaf Stapledon’s Last and First Men … and an essay in J.B.S. Haldane’s Possible Worlds both of wh[ich] seemed to take the idea of such [space] travel seriously and to have the desperately immoral outlook” — an outlook that he tried to “pillory” in the character of the villain of his story.

Despite the morally inflected nature of Lewis’s stories, they were fully accepted as science fiction by his contemporaries. Madeleine L’Engle’s A Wrinkle in Time had similar moral themes, and also included animistic elements in the universe (the stars themselves joined the battle against evil, for example). Yet she described her work at the time of publication as “science fiction.” Consider also the original Battlestar Galactica series, which had non-naturalistic themes and elements, but which was thought of at the time as wholly science fiction. The distinction  between science fiction and fantasy then rested on aesthetics instead of metaphysics.

However, over time, the borders of science fiction have become coextensive with the borders of naturalism. Today, Lewis’s Space Trilogy and L’Engle’s A Wrinkle in Time are considered science fantasy, and Battlestar Galactica is treated as science fiction with fantasy elements (a caveat that indicates it is not pure science fiction). This shift in the contours of science fiction has, we think, been part and parcel with the maturing of the genre over time. Now, the distinction between science fiction and fantasy rests on metaphysics instead of aesthetics, and our collective intuitions tend to confirm this, even when we have not always been able to articulate that distinction.

In conclusion, today, science fiction is speculative fiction that is set in a naturalistic universe, while fantasy is speculative fiction set in a non-naturalistic universe. The distinction is asymmetric: Including fantasy elements in an otherwise science fiction universe pivots the work towards fantasy, even if the rest of the work is heavily inflected with science fiction aesthetics and themes. In this way, non-naturalistic elements are like food coloring in water — they turn the entire work into fantasy, including the science fiction bits; unless the work already has overwhelming inertia towards science fiction, non-naturalistic elements will change readers/viewers’ perception of the work.

The same is less true for science fiction elements. Including science fiction elements in an otherwise fantasy universe does not make the work science fiction (although it may brand it as science fantasy). Turning an established fantasy into science fiction requires retconning all formerly non-naturalistic elements, and is a much harder of a task. The reason it was so easy to do with Star Wars was because the Force — and its partial nature — was the non-naturalistic element of the franchise, and was relatively easy to retcon.

Our distinction presents a more constrained and expanded view of science fiction: The work must involve a wholly naturalistic universe to be science fiction; mere aesthetics are not enough. At the same time, the naturalism doesn’t have to be our naturalism, and the rules don’t have to be our rules. This helps us to be more thoughtful in how we blend science fiction and fantasy. It’s OK to have characters act like scientists in a fantasy world, so long as the rules themselves don’t all fall into naturalistic grooves. Furthermore, it’s also perfectly fine to craft an alt-universe science fiction world. The risk and danger exists mainly when authors do not understand their own worldview.

And further, we argue that genuine science fantasy is an underexplored genre. Writers creating worlds with a science fiction aesthetic often over-naturalize the fantastic in their stories; they attempt to signal — even if inadvertently — that the extraordinary things that happen are wholly naturalistic. We would welcome more stories that have a thoroughly science fiction aesthetic, but which are genuinely morally inflected, or in which non-naturalistic themes and assumptions saturate the world and its characters, or at least play a significant role. This is, in some ways, what made the original Star Wars trilogy so great. Part of our reason for writing this series is to express a craving for this sort of story, and for such stories to be unabashedly non-naturalistic, rather than apologetically or reluctantly so, with halting attempts to signal naturalism along the way.