It must be remembered that we live in an entirely new world. Two hundred years ago, stories of this kind were not possible. Science, through its various branches of mechanics, electricity, astronomy, etc., enters so intimately into all our lives today, and we are so much immersed in this science, that we have become rather prone to take new inventions and discoveries for granted. Our entire mode of living has changed with the present progress, and it is little wonder, therefore, that many fantastic situations - impossible 100 years ago- are brought about today. It is in these situations that the new romancers find their greatest inspiration.
- Hugo Gernsback, Amazing Stories Magazine No. 1, April 1926. (Read the entire issue here.)
That's... a very astute definition of science fiction, I think. Gernsback was a bit over the top at points, but I think that hits the nail on the head for the type of stories that defined the genre. Must file this away for later.