“Science Fiction has rivets, fantasy has trees.”

The Twilight Zone, "The Fugitive" (1962).
The Twilight Zone

The Twilight Zone, "The Fugitive" (1962).
The Twilight Zone
Variant: Science fiction is the improbable made possible, and fantasy is the impossible made probable.
Context: It is said that science fiction and fantasy are two different things. Science fiction is the improbable made possible, and fantasy is the impossible made probable.

The Circus of Dr. Lao Introduction (1956)

It's the art of the possible. Science fiction is the art of the possible. It could happen. It has happened.
A Conversation with Ray Bradbury - Point Loma Nazarene University, Writer's Symposium By The Sea; April, 2001 http://www.cosmolearning.com/videos/a-conversation-with-ray-bradbury-2001-1131/

Source: Letters to a Young Scientist (2013), chapter 5, "The Creative Process", page 69.

Interview with Weird Tales (24 May 2007) http://weirdtales.net/wordpress/2007/05/24/george-rr-martin-on-magic-vs-science/
Context: I think that for science fiction, fantasy, and even horror to some extent, the differences are skin-deep. I know there are elements in the field, particularly in science fiction, who feel that the differences are very profound, but I do not agree with that analysis. I think for me it is a matter of the furnishings. An elf or an alien may in some ways fulfill the same function, as a literary trope. It’s almost a matter of flavor. The ice cream can be chocolate or it can be strawberry, but it’s still ice cream. The real difference, to my mind, is between romantic fiction, which all these genres are a part of, and mimetic fiction, or naturalistic fiction.

The Guardian, London (7 November 1988)

In a panel about his work in Comic Con 2010. Quoted in China Miéville Takes Comic Con http://www.tor.com/blogs/2010/08/china-mieville-takes-comic-con.