“It’s never ‘should’ with engineers, my old friend, but ‘how’? Have you not learned that much?”

Source: The Warrior Within : The Philosophies of Bruce Lee (1996), p. 117

Diary entry of Friday (2 February)
Source: Nausea (1938)

Not found in Twain's works, this was attributed to him in Reader's Digest (September 1939): no prior attribution known. Mark Twain’s father died when Twain was eleven years old.
Disputed
Variant: When I was a boy of fourteen, my father was so ignorant I could hardly stand to have the old man around. But when I got to be twenty-one, I was astonished at how much the old man had learned in seven years.

Source: Invisible Man

Source: The Coldest Girl in Coldtown

Source: Eragon

Review of “Eyes of Amber”, by Joan D. Vinge (as anthologized in New Women of Wonder, edited by Pamela Sargent http://jamesdavisnicoll.com/review/yet-more-sf-about-women-by-women, 2015
2010s
Context: There’s a rule I used to call The Niven Rule but which I just now have decided to call the Rusting Bridges rule. It came to me after reading Niven’s “All The Bridges Rusting.” In this story, humans have by the early 21st century explored the Solar System and sent not just one but two crewed ships to Alpha Centauri … despite which the characters moan endlessly about the dire state of the space program. “Eyes of Amber” would be another example of the Rusting Bridges [Rule]: No matter how much the space program you actually have has achieved, whether it’s first contact with aliens or trips to nearby stars, it can never have achieved as much as the space programs you can imagine would have achieved in its place, given that imaginary programs aren’t limited by issues of politics, funding, or engineering.

2000s, Kingdom of Fear: Loathsome Secrets of a Star-crossed Child in the Final Days of the American Century (2004)