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)    

H. G. Wells:

Make one fantastic assumption (such as positing that a time machine exists) and extrapolate everything else from that.

  robot-thinker