Why are SF and Fantasy so often grouped together? Obviously, because they share readership and so are well placed together in book stores. Fantasy is the mother genre -- e.g. Gilgamesh, the Illad, Odyssey and most religions. Sci Fi is the brash offshoot. All literature has deep roots in fantasy, which in turn emerges from the font of our dreams. Having said that, what is my definition of the separation? I think it is very basic, revolving around the notion of human improvability. Science Fiction considers the possibility of learning and change.