Fantasy and science fiction are the dominant categories under the broad speculative fiction banner, but what’s the difference between them? I started wondering about that when I re-read Coleridge’s The Rime of the Ancient Mariner, which was a source for Jacinta’s Lovers.

The Rime is one of my favourite poems, but a couple of verses have always puzzled me—

FIRST VOICE
‘But why drives on that ship so fast,
Without or wave or wind ?’

SECOND VOICE
‘The air is cut away before,
And closes from behind.’

The Polar Spirit’s fellow-demons, invisible inhabitants of the air, are having a chat here, and in effect they’re giving the reader an explanation for the motion of the ancient mariner’s vessel.

These creatures are magical and fairy-like,¹ so one puzzle is why they need to discuss the ship’s motion at all—it’s just more magic. But more importantly, why is Coleridge bothering to offer an explanation to the reader anyway?

plausible explanations

The Rime of the Ancient Mariner is fantasy, not sci fi. There are a quite a few magical occurrences in the poem, and by and large Coleridge makes no attempt at explanations.

For me, that’s a key difference between the two genres. In sci fi, an explanation for weird stuff is usually given or at least implied, and it must be plausible to some degree, although exactly how plausible is open to debate.

Returning to the two demonettes, there’s no talk of worm-holes or warp factors, but it’s still a sci fi explanation. Exaggerating just a little, their conversation could continue—

FIRST VOICE
‘Cool. So there’s a partial vacuum at the front of the ship and an increased pressure behind it.’

SECOND VOICE
‘Yeah, it’s super cool. The pressure difference is almost fifty kilopascals.’

Although the explanation Coleridge gave is implausible, it makes that section of the poem read like sci fi rather than fantasy, simply because he attempted to justify the ship’s motion.

schrödinger’s quantum wand

In speculative fiction, a number of concepts/creatures/devices have appeared so often that they’ve become standards. In sci fi, teleportation, time machines, subspace drives, and so on, now appear with very little explanation. On the other hand, if you’re reading about elves, gremlins, and magic wands, then it’s probably fantasy.

However, if a device has some sort of technical explanation,² even one as fantastically commonplace as a magic wand, then you’re back with sci fi. For example,

The two demonettes are having coffee. An old man with a long grey beard and a glittering eye approaches and starts mumbling about a sailing ship. He abruptly vanishes.

FIRST VOICE
‘Cool wand.’

SECOND VOICE
‘Yeah, it’s super cool. It has a built-in motion detector with a subspace link to a quantum teleporter.’

¹The illustration is adapted from one of Gustave Doré’s engravings for the 1876 edition of The Rime, and it shows his vision of the Polar Spirit’s demonettes.

²For sci fi, explanations have to be plausible or real to some extent. For example, Terry Pratchett’s description of the discworld iconograph (camera), which has an imp inside who paints the scene outside, is a continuation of his amazing fantasy world, and not sci fi.