In an article published in the journal CLCWeb:Comparative Literature and Culture, Simone Caroti argues that "cognition" is what sets science fiction apart from fantasy, using Greg Bear's Blood Music as an example.
The act of cognition, of rationally making sense of – and coming to terms with – the estranging elements, increases the sense of wonder inherent in [science fiction], whereas it destroys the pleasure of reading [fantasy]. Magic as represented by writers like Tolkien is best left unexplained, because it belongs to the realm of the irrational. Like a fairy, it is a fragile thing, and trying to rationalize it or explain it away will kill it. On the other hand, a rationally constructed estranging element thrives on cognition, as will readily become apparent when a typical example of the genre is examined. Greg Bear's Blood Music (1985) is set in our times, and at the beginning of the novel no difference from the world we know is offered. However, estrangement soon rears its head in the form of a biologist's development of a new strain of sentient bacteria. When the private lab he is working for cuts his funds and fires him, deeming his experiment illegal and dangerous, this modern-day Victor Frankenstein injects himself with he latest batch of his creations and goes away. In only a few days, these bacteria spread from their original host to contaminate half the population of the planet. As the novel nears its completion, the world has indeed become estranged from what the reader is used to, but this is nothing compared to the discovery lying in wait at the very end, when the true nature of this biological agent is revealed. Far from being just another outlandish example of malevolent disease (like the monstrous alien virus in John Carpenter's 1982 film The Thing), these bacteria have in fact evolved into a completely new life-form inhabiting an entirely different plane of existence, and have taken with them all the human beings who were thought dead. After finishing the last page, the sense of wonder is still with us, even stronger than before.
Caroti's conclusion - that plausible scientific revelations in science fiction are part of what makes the genre entertaining – is one I agree with. As he puts it:
The cognitive discovery of the new life-form's true nature implies a series of revelations regarding our understanding of reality and our place in the universe. Far from diminishing our sense of wonder, these revelations greatly increase it, first of all by grounding its presence within a plausible rational framework, and then by extending the implications of this framework far beyond what we had at first imagined.
I'm just not sure that "cognitive discovery" universally separates science fiction from fantasy.
There are many science fiction novels in which the "cognitive discoveries" have little basis in real science, and sometimes are difficult to distinguish from magic. I haven't read that much fantasy, but it also rings false to me that magic is necessarily always "unexplained" and "irrational", at least in the context of the story.
I think there's a great gray area between Tolkien's fantasy and Bear's hard science fiction, which includes science fiction with a seemingly fantasy setting like the McCaffrey's Pern novels or Wolfe's Book of the Long Sun, space operas that are focused on adventure and space battles, science fictional heroes with amazing mental or physical powers who seem to differ from wizards in name only, and stories with science fictional settings like Le Guin's The Dispossessed that explore political systems, rather than science or technology.
It's kind of fun to try to imagine the classics of fantasy rewritten as science fiction. In the Lord of the Rings the One Ring could have been fabricated in a laboratory by a mad genius scientist, rather than forged in the heat of a volcano by a powerful wizard. In the Harry Potter novels, the horcruxes would be computer devices for storing copies of Lord Voldemort's uploaded mind, rather than magical devices used to hide "a part of his soul for the purpose of attaining immortality". But while the plot and characters might stay essentially the same in such a re-genred novel, that shift from magical devices to objects developed through science and technology represents a significant difference in world view.
What sets science fiction apart from fantasy is not just the tropes of the genre – future or extraterrestrial setting, space travel, advanced technology or scientific discoveries. It also the underlying assumption that the universe is controlled by natural mechanisms, rather than supernatural forces. Even though a sufficiently advanced technology may be indistinguishable from magic, such a technology must have been created through the application of scientific principals. And as speculative as that technology or the science it is based on might be, it's a feat that humans could hope to achieve. Magic, on the other hand, can exist only in the realm of fiction.
So I agree with Caroti that science fiction uses a "plausible rational framework" to support the speculative aspects of the story. Where I think we disagree is whether the "cognitive discovery" that's central to hard science fiction stories like Bear's Blood Music is also a necessary part of SF, or if the framework alone is enough to set the genre apart from fantasy. I'd argue the latter.
Caroti, Simone. "Science Fiction, Forbidden Planet, and Shakespeare's The Tempest." CLCWeb: Comparative Literature and Culture 6.1 (2004): http://docs.lib.purdue.edu/clcweb/vol6/is...