Outer Spaces

The major distinction between fantasy and science fiction is, simply, that science fiction uses one, or a very, very few new postulates, and develops the rigidly consistent logical consequences of these limited postulates. Fantasy makes its rules as it goes along… The basic nature of fantasy is “The only rule is, make up a new rule any time you need one!” The basic rule of science fiction is “Set up a basic proposition–then develop its consistent, logical consequences.

John W. Campbell