Not only does the future have the great advantage of being not-here , but it is a place where the powers of science and technology will be greatly extended. The future is the Middle Earth of science fiction. It accounts for the relationship of fantasy and science fiction and provides a means for telling them apart - a test that seems rreasonably adequate. It explains why the future is so associated with science fiction but not insist that science fiction must be associated with the future. The only real complaint I have with it as a definition is that the wording is not sharp.
Speculative fiction is another branch of fantasy.
It is, or can be, closely related to science fiction but it is a different breed of animal. The above essay is overly long and is also incomplete. The idea, however, is simple. Prior to the enlightenment there was no real conception of the future as being different other than religious visions. With the industrial revolution came the idea of progress - the future would be like the present, only there would be more and it would be better.