This discussion is one that’s still in progress—and probably will be for some time to come--so the boundaries aren’t clearly marked. Still, many seem to agree that possibility is a determining factor. Science fiction explores what is possible (even if it’s improbable), while fantasy explores the impossible. Of course, possibility comes with some measure of subjectivity, which is what complicates matters.
Let’s turn to Ray Bradbury, author of, among other great novels, Fahrenheit 451 and The Martian Chronicles, who has written in both genres. He described science fiction this way:
Science fiction is really sociological studies of the future, things that the writer believes are going to happen by putting two and two together . . . Science fiction is a logical or mathematical projection of the future.
Bradbury’s novel Fahrenheit 451 reveals an America where people are consumed by media and books are outlawed and burned when found. Indeed, this is chillingly possible: books can be burned. Bradbury also imagined technology to support this world, including the mechanical hound, an eight-legged invention that finds individuals based on smell, and the spot-wavex scrambler, which allows television viewers to participate in what is unfolding on screen. There’s possibility, here, too. In fact, today’s flat screen televisions seem to be a significant step toward Fahrenheit 451’s TV Parlor. Science fiction usually has a scientific premise at its core. For example, Jurassic Park, a novel by Michael Crichton, explores cloning.
While science fiction draws on and extrapolates from what we know about reality and science, fantasy invents what does not (and likely could not) exist in our reality. J.R.R. Tolkien’s The Lord of the Rings takes place in the invented Middle-earth, a place populated by hobbits, dwarves, elves, goblins and more. It features rings that are powerful and One Ring that could control the power of all the others. There are plenty of other fantasy realms. Anne Rice, for example, explores vampires in her series of novels, The Vampire Chronicles.
Some novels draw from both and don’t easily fall under one category. Some call this category science fantasy. Others debate which traditional category—science fiction or fantasy—the book should fall under and why. Categories do have their importance, but don’t feel hemmed in by them when you’re writing. Follow the characters and the action on the page and see where it takes you.