1.
Life moves faster, and we move with it or are left behind. We ride the back of galloping technology, and we cannot dismount without breaking our necks.
2.
Science fiction is based upon the belief that the world is changing, that the way we live is changing, and that humanity will adjust to it, or will adjust change to humanity, or will perish.
3.
The wellsprings of science fiction are in the mind; of literature, in the heart. Science fiction appeals to the intellect and achieves its effects through the tensions created by treating an imaginative concept naturalistically, through the disproportion observed between humanity and the universe, between people's lifetimes and eternity, between their accomplishments and their dreams; literature appeals to the emotions and achieves its effects through the analysis of character and the interplay of language.
Science fiction and literature seem poles apart. Fantasy and literature, on the other hand, are inseparable; fantasy, which is concerned with the conflict between humanity and its imagination, which deals with the fanciful explanations humanity has created to rationalize itself, its origin, and its fate, and the mysterious forces that act upon it and its world-that is, with myth and legend-has as long a history as literature itself; they are interwoven and in some ways identical.
Insofar as we dare to generalize about so diverse and so undefinable a phenomenon, we might say that science fiction is a social vision, a public vision; fantasy is always personal and private.
Science fiction concerns man's dreams of reality; fantasy, the reality of man's dreams.
4.
Campbell [Analog magazine editor] pointed out that everything necessary to predict an atomic weapon, even an atomic bomb, was available in scientific literature prior to 1939. All science-fiction writers had to do was read the right journals. In 1946 Campbell would write that science fiction exists in the gap between the discovery of a scientific fact in the laboratory and its engineering application. [Good definition of hard science fiction I think].