Jack Williamson

"Hard" science fiction ... probes alternative possible futures
by means of reasoned extrapolations in much the same way that good
historical fiction reconstructs the probable past. Even far-out fantasy
can present a significant test of human values exposed to a new 
environment. Deriving its most cogent ideas from the tension between
permanence and change, science fiction combines the diversions of novelty
with its pertinent kind of realism.