Defining Science Fiction

George Hay ("futurologist")    
Brian Aldiss (science fiction writer)    
H. G. Wells (polymath)    
Eric Rabkin (tri-partite definition)    
Frederic Brown (science fiction writer)    
Eric Rabkin (theoretical definition)    

Frederic Brown (a paradigmatic definition):

"After the last atomic war, Earth was dead; nothing grew, nothing lived. The last man sat alone in a room. There was a knock on the door...."

  robot-thinker