"It's been said that science fiction and fantasy are two different things; science-fiction the improbable made possible, fantasy the impossible made probable."
— Rod Serling, The Twilight Zone, Season 3, Episode 90, "The Fugitive"
"In fantasy, impossible things exist. In science fiction, impossible things exist and can be understood by humans. In supernatural horror, impossible things exist and cannot live in peace with humans."
— Will Shetterly, writer.
"Science-fiction works hand-in-glove with the universe. Fantasy cracks it down the middle, turns it wrong-side-out, dissolves it to invisibility, walks men through its walls, and fetches incredible circuses to town with sea-serpent, medusa, and chimera displacing zebra, ape, and armadillo. Science-fiction balances you on the cliff. Fantasy shoves you off."
—Ray Bradbury, writer.
"The best kind of science fiction involves science."
"Fantasy deals with the immeasurable while science-fiction deals with the measurable."
—Walter Wangerin, Jr., fantasy writer.
"A lot of science fiction is fantasy with nuts and bolts painted on the outside."
"The difference between fantasy and science fiction is that in fantasy, dragons can hover; in science fiction, they can not."
—Seen on a button
"In fact, I would define science fiction and fantasy not in terms of fidelity to current scientific dogma but in terms of divergent attitudes to ideas and explicability: where fantasy writers invoke mystic archetypal imagery, science fiction writers map novel conceptual territory."
—Justin B. Rye
"Science fiction has rivets, fantasy has trees."
If there's a zeppelin, it's alternate history. If there's a rocketship, it's science fiction. If there are swords and/or horses, it's fantasy. A book with swords and horses in it can be turned into science fiction by adding a rocketship to the mix. If a book has a rocketship in it, the only thing that can turn it back into fantasy is the Holy Grail.
— Debra Doyle
Science fiction expands our world; fantasy transcends it.
—Mark Wilson, here.
"Science Fiction is just Fantasy with a space program and a proper understanding of physics."