“Einmal ist keinmal, says Tomas to himself. What happens but once, says the German adage, might as well not have happened at all. If we have only one life to live, we might as well not have lived at all.”

Tomorrow's Children (p. 30)
Short fiction, The Book of Poul Anderson (1975)

Source: Glasshouse (2006), Chapter 13, “Climb” (p. 224)

Book III, ch. 4.
The Japanese Family Storehouse (1688)

Source: Dexter in the Dark

Source: The Body in the Library

The Man Without Qualities (1930–1942)
Variant: If there is a sense of reality, there must also be a sense of possibility. To pass freely through open doors, it is necessary to respect the fact that they have solid frames. This principle, by which the old professor had lived, is simply a requisite of the sense of reality. But if there is a sense of reality, and no one will doubt that it has its justifications for existing, then there must also be something we can call a sense of possibility. Whoever has it does not say, for instance: Here this or that has happened, will happen, must happen; but he invents: Here this or that might, could, or ought to happen. If he is told that something is the way it is, he will think: Well, it could probably just as well be otherwise. So the sense of possibility could be defined outright as the ability to conceive of everything there might be just as well, and to attach no more importance to what is than to what is not.

Source: The Sons of Heaven (2007), Chapter 22, Section 1 “Child Care in the Cyborg Family, Volume Six: The Challenge of Psychological Development” (p. 268)

Source: Heaven to Betsy