“The future is there… looking back at us. Trying to make sense of the fiction we will have become.”

The Paris Review interview (2010)
Context: I often use the metaphor of Perseus and the head of Medusa when I speak of science fiction. Instead of looking into the face of truth, you look over your shoulder into the bronze surface of a reflecting shield. Then you reach back with your sword and cut off the head of Medusa. Science fiction pretends to look into the future but it’s really looking at a reflection of what is already in front of us.

Attributed to an interview on Larry King Live; also quoted in Quotable Quotes (1997) edited by Deborah Deford
Attributed variant: The difference between fiction and reality is that fiction has to make sense.
Clancy here expresses an idea evoked in similar statements made by others, all derived from the orignial made by Lord Byron:
Lord Byron: Truth is always strange; stranger than fiction.
Mark Twain: Truth is stranger than fiction, but it is because fiction is obliged to stick to possibilities, truth isn't.
G. K. Chesterton: Truth must necessarily be stranger than fiction, for fiction is the creation of the human mind and therefore congenial to it.
Leo Rosten: Truth is stranger than fiction; fiction has to make sense. (attributed)
1990s

Sec. 624, as translated by Tobias Dantzig in Number, the Language of Science. Fourth edition, New York: Doubleday 1954, p 141. See discussion of this entry for details.
The Will to Power (1888)

"John Rivers" in The Genius and the Goddess (1955)
Source: The Genius And The Goddess

Source: Red: Passion and Patience in the Desert

Introduction
Naked Economics (rev. and updated ed., 2010)

Source: On the Jellicoe Road