You've got fantasy on my science fiction

A number of discussions in LJ and other blogsplots on defining the genres. I'm more of a seat of the pants person-- "I know it when I see it," but aireon shared a definition that I really like.

A science fiction universe is one in which the universe is finite, that is, it is assumed that all things can, in the end, eventually, even if not within the time frame of the story, be understood (measured, quantified, etc).

A fantasy universe is one in which the universe is infinite; there will always be things that can't be understood.