To me, the fundamental difference in science fiction and fantasy novels has a lot to do with purpose. Science fiction seems to be an attempt at defining humanity, what it means to be human, and the struggle with the results of human creativity. Fantasy, on the other hand often tends to be focused on the ideas of good and evil, of a penultimate battle between powers, in which people play only a small part or, serve as a reflection of the greater war. These two genres seem to reflect the conflict in society between science and religion; that which we can see, feel, touch, and that which requires faith. My favorite authors always manage to tangle the two so well that it is hard to pick a classification, because in reality there is rarely any clear delineation.