Mohs Scale of Science Fiction Hardness/Quotes

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.

—James Nicoll

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.

There has always been a strain of sorcery in science fiction. Whether the thinly-veiled magic of space monks with laser swords, the once-serious belief in psychic powers and untapped human mental abilities, or the inevitable compromises with known physical reality necessary to allow faster-than-light space travel, there has always been a niche for the impossible. Only the most rigorous, diamond-hard sci-fi is entirely free of witchery, and that often for no longer than it takes for scientific discovery to leapfrog the text’s assumptions.

—Kevin Crawford, Codex of the Black Sun: Interstellar Sorcery and Magic for Stars Without Number