“It is the game of “what if?”. You take that which is known, and you extrapolate – and you keep it within the bounds of logic, otherwise it becomes fantasy – and you say, “Well, what if?”. That’s what speculative fiction is, and at its very best, it is classic literature, on a level with Moby Dick and Colette and Edgar Allan Poe.”
–Harlan Ellison
From The Guardian Q&A:
http://www.theguardian.com/books/2013/jun/14/harlan-ellison-q-and-a-interview