I will define science fiction, first, by saying what science fiction is not. It cannot be defined as 'a story set in the future,' [nor does it require] untra-advanced technology. It must have a fictitious world, a society that does not in fact exist, but is predicated on our known society... that comes out of our world, the one we know:
This world must be different from the given one in at least one way, and this one way must be sufficient to give rise to events that could not occur in our society…
There must be a coherent idea involved in this dislocation…so that as a result a new society is generated in the author's mind, transferred to paper, and from paper it occurs as a convulsive shock in the reader's mind, the shock of dysrecognition.

[In] good science fiction, the conceptual dislocation---the new idea, in other words---must be truly new and it must be intellectually stimulating to the reader…[so] it sets off a chain-reaction of ramification, ideas in the mind of the reader; it so-to-speak unlocks the reader's mind so that that mind, like the author's, begins to create…. The very best science fiction ultimately winds up being a collaboration between author and reader, in which both create---and enjoy doing it, [experiencing] the joy of discovery of newness.

--Philip K. Dick, The Collected Stories of Philip K. Dick, Carol Publishing, 1999, xviii-xiv.