Quotes: Mohs Scale of Science Fiction Hardness

Mind you, anything written by Doc Smith tends to be rather light on plausibility. You know what "hard science fiction" is, right? Well, Smith's work is so soft it squishes.
Ward, rec.games.frp.gurps, 5/1/2002

Soft SF says "the Go Fast Now drive works because I pressed that red button". Hard SF takes the time to explain things in enough detail that the reader can work for themselves why it is the drive can't possibly work that way.

Isaac: Interesting. What's the explanation for how it actually works?
Warren: The hyperdrive utilizes IJD technology.
Isaac: Inter...dimensional jump? IDJ?
Warren: IJD. "It Just Does".
Absurd Notions, pg. 111 (2003).

"Il invente!" ("He makes it up!")
—Attributed to Jules Verne, regarding the softer science fiction of H. G. Wells.