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To be science fiction, the story must depend upon fictional science or scientific achievements. Although that is most plausible for the future, it could be set in the present or past; e.g. we could...
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#4: Attribution notice removed
Source: https://writers.stackexchange.com/a/45066 License name: CC BY-SA 3.0 License URL: https://creativecommons.org/licenses/by-sa/3.0/
#3: Attribution notice added
Source: https://writers.stackexchange.com/a/45066 License name: CC BY-SA 3.0 License URL: https://creativecommons.org/licenses/by-sa/3.0/
#2: Initial revision
To be science fiction, the story must depend upon fictional science or scientific achievements. Although that is most plausible for the future, it could be set in the present or past; e.g. we could push some modern, actual scientific achievement back into pre-history; e.g. the Chinese invented gunpowder in 700 AD, it took them 200 years to realize it could be weaponized. They also invented quench-hardened steel, about 400 AD, and had a weaker form of steel around 200 BC. So what if some genius had invented actual **rifles** accurately firing bullets about 700 AD? That story would be science fiction, set in the distant past. A story simply set in the future, where the plot is not driven by scientific developments, even if the story uses plausible scientific developments; would be speculative fiction, not science fiction.