The problem is, Damon Knight doesn't go far enough. The truth is that science fiction is far more than what he points to when he says it.
OK, here's how it is:
1. Science Fiction is defined as fiction which has some element of the fantastic in it.
2. the differences between science fiction, fantasy, sci-fi, SF, spec fic, paranormal fiction, and whatever else you may have, are all basically cosmetic and arbitrary.
And now for the explanations.
1. Any element of the fantastic turns an ordinary piece of fiction into science fiction. A bit of magic, some telepathy, some new tech, a visit from a god, all these are things which make an ordinary story into the stuff that I want to read.
Some examples:
The Left Behind Series, generally classified as religious fiction, is actually part of the stuff I read, since the occurence of miracles makes it science fiction.
Just about anything by Michael Crichton is science fiction, from The Andromeda Strain, to Jurassic Park. I don't really have to explain this, do I?
The DaVinci code is science fiction for a couple of reasons. One is that it deals with secret societies, which may or may not exist. Just like we sometimes read about magic or telepathy, which may or may not exist. The second reason is that the characters have a distant connection with, well, a god of some sort. OK, that's a bit of a stretch, but the first part certainly fulfills the fantastic requirement.
Obviously, a lot of the movies that make a lot of money are science fiction. All the superhero movies, for instance.
2. Some of you may take exception with me for saying that science fiction and fantasy are the same thing. One of the reasons that I hold to this point of view is that the only hard definition of fantasy is the same as the one for science fiction. That is, #1 up there. Most of you can point to something you call fantasy and have most people agree with you, but popularity is not reality. Some stories mix these supposed genres together, like Star Wars, where you got your spaceships and lasers mixed up with my swords and sorcery (obligatory Reeces reference there). So is it fantasy or SF? Nonsense, it's science fiction, the stuff I read.
In the case of Fantasy, or SF, or Pet Fantasy, or Paranormal Romance, or Military Sci-Fi, the definitions are fast and loose, and they are defined by what you point to when you say it.
You may now all flame me.