And many authors wrote both and the fantasy was often given a cursory scientific explanation. Much of the science fiction featured esp or other "Psi" abilities. Take Marion Zimmer Bradley's Darkover series or Andre Norton's Witch World books. They're SF but read like fantasy much of the time.
There are books that are definitely one or the other but the boundary between the two is vague. Fantasy didn't really evolve as a separate genre until the 70s.
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