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I have been cautioned against blending:

• Traditional fantasy elements

Such as magic systems and exotic, less plausible creatures (on a scientific level - magic tends to explain away these beasts)

• Traditional sci-fi elements

Such as advanced technology and civilizations amidst the stars.

I have taken it upon myself to harmonize the two in my current worldbuilding project. I know I cannot be the first. I love the creativity found in both, and it is going well so far. I have been exploring the potential for humanity with both tools at their disposal. (Magic and science, essentially)

Why do people advise to stick to one or the other?
I encountered this on a video specifically dealing with magic systems, but he did not elaborate.

Posted this first on Worldbuilding, and was instructed to try it here. My apologies, still green to Stack.

The reason this is often recommended against is because by mixing them you find yourself unable to meet certain genre conventions. Fantasy readers want swords and lords, sci-fi readers want spaceships and aliens. Even more importantly fantasy readers want epic tales of good versus evil, while sci-fi often want to grapple with trans humanist ideas. But trying to match both markets, you match neither.

That said, I don't think it is very good advice. It is not the writer's job to market, it is their job to write a good story. If you have a good story that involves fantasy and sci-fi elements, write it.

Plenty of existing works mix genres to varying degrees and been successful. Star Wars is mostly sci-fi with a little fantasy, while the Shannara books are fantasy with some subtle sci-fi.

Mixing genres can be a challenge, but don't limit yourself by blindly following a rule.

You are correct that you are not the first to attempt this blending. The entire Urban Fantasy genre does an admirable job of blending today's scientific reality (which was yesterday's science fiction) with classical fantasy elements. Many science fiction subgenres such as Gothic SciFi, Space Horror and Slipstream exist specifically to categorize different approaches to such blending. Best selling authors such as Dean Koontz, Steven King, Raymond Feist, Mercedes Lackey, Clive Barker, Neil Gaiman, Kim Harrison, Laurell Hamilton, Peter Straub, S.M. Sterling and George Martin (along with many many others) have at least one book which bridges the gap between fantasy and science fiction.

Whoever cautioned you against blending is ignoring this major trend in the modern fiction market. With all due respect, I couldn't disagree with them more.

There are challenges to successfully merging fantasy and science fiction. As @RTPax noted, there are genre conventions which must be respected as you mix your story together. But those conventions are not mutually exclusive. There are many scientifically plausible explanations for magic existing in an apparently physical-law-limited world. Mercedes Lackey suggests in her Razor's Edge series that the Elves have been hiding until just recently, when the rise of aluminum and alloy metals reduced the amount of (fatal to elves) raw iron exposed in our day to day lives. S.M. Stirling blames us humans for breaking our previously functional universe though our high energy physics experimentation, opening the door for all types of unexplainable events. You will need to find a reason for why magic now exists in your scientific world, or why the world still believes in science in the presence of magic which defies cause and effect.

Writing a merge is not easy. It requires that you wear two hats while you are writing so that the requirements of each genre can be filled. You must also be careful during editing, not to leave that hard-earned fulfillment on the cutting room floor.

But if it is what you are called to write, and if you are up for the challenge, the results can be spectacular.

Keep Writing!

A lot of wonderful books combine both science-fiction and fantasy.

And why not?

If magic can exist in books set in the modern age or in the past, why not in books set in an otherwise realistic future?

Why can't mythical creatures go to space?

What reason is there to ban ghosts and purveyors of the supernatural from a world of great technology?

One of the most common crossovers involves time travel. Time travel can be either science-fiction or fantasy, depending on how it's used and what other elements are present. If you're Wesley Chu (Time Salvager), it's 100% science-fiction. If you're Deborah Harkness (A Discovery of Witches), it's absolutely fantasy.

Why do people advise against blending the two?

My guess is because most readers who enjoy one genre eschew the other. Or at least that's the belief. I love both and so do a lot of readers. But if you write both together, you run the risk of turning off a subset of potential readers. Of course, you also have a lot to gain from crossover readers.

Write what you love. With luck, the audience will follow.

Examples: Dune (series), Ender's Game (series), The Bone Season (series), The Golden Compass (series), A Wrinkle in Time, Star Wars (movie series).