“Fiction has to be plausible. All history has to do is happen.”

Source: Theses on the Philosophy of History (1940), III
Context: Nothing that has ever happened should be regarded as lost for history. To be sure, only a redeemed mankind receives the fullness of its past — which is to say, only a redeemed mankind has its past become citable in all its moments. Each moment it has lived becomes a citation à l'ordre du jour — and that day is Judgement Day.

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.

As quoted in "The Gentle Philosopher" (2006) by John Little at Will Durant Foundation
Context: Perhaps the cause of our contemporary pessimism is our tendency to view history as a turbulent stream of conflicts — between individuals in economic life, between groups in politics, between creeds in religion, between states in war. This is the more dramatic side of history; it captures the eye of the historian and the interest of the reader. But if we turn from that Mississippi of strife, hot with hate and dark with blood, to look upon the banks of the stream, we find quieter but more inspiring scenes: women rearing children, men building homes, peasants drawing food from the soil, artisans making the conveniences of life, statesmen sometimes organizing peace instead of war, teachers forming savages into citizens, musicians taming our hearts with harmony and rhythm, scientists patiently accumulating knowledge, philosophers groping for truth, saints suggesting the wisdom of love. History has been too often a picture of the bloody stream. The history of civilization is a record of what happened on the banks.

Source: " A Case of Voluntary Ignorance http://www.christiebooks.com/ChristieBooksWP/2013/11/a-case-of-voluntary-ignorance-by-aldous-huxley/" in Collected Essays (1959)

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