People who come here often may remember my definition of science fiction, viz.
Science fiction is a genre in which it is considered true that only reason and the assumption that the physical world is the only one that exists, can explain the universe, and in which the story typically but not necessarily, features science and/or technology.
It just struck me that there is a corollary to this which I’ve always known but can now explain; certain genre mixes involving science fiction do not work. You can have some great mixed-genre offerings that include sci-fi such as sci-fi and crime (The Caves of Steel – nuff said), sci-fi and romance (The Left Hand of Darkness), sci-fi and historical (Galileo’s Dream), and so on. However, what never ever works for me is sci-fi and fantasy. Ever. The end result is pure fantasy with some science and technology thrown in – not sci-fi at all.
It follows from the definition, of course, that this should be so. Fantasy by definition requires a story in which something more than the physical world must exist (that’s why they call it fantasy!) so if you add that to sci-fi (by my definition) you break the key requirement of the genre and no longer have sci-fi at all. It’s like clean and dirty: once they’re mixed, all you have is dirty.
Just thought I’d mention it.