Frost snorted. “I certainly do—if he has observed it with his own eyes and ears, or gets it from a source known to be credible. A fact doesn’t have to be understood to be true. Sure, any reasonable mind wants explanations, but it’s silly to reject facts that don’t fit your philosophy.”
Elsewhen (pp. 161-162)
Short fiction, Off the Main Sequence (2005)

“Imperialism degrades both oppressor and oppressed.”

Robert A. Heinlein book Solution Unsatisfactory

Solution Unsatisfactory (p. 98)
Short fiction, Off the Main Sequence (2005)

“It is very difficult to tuck a bugle call back into a bugle. Pandora’s Box is a one-way proposition. You can turn a pig into sausage, but not sausage into pig. Broken eggs stay broken.”

Robert A. Heinlein book Solution Unsatisfactory

Solution Unsatisfactory (p. 67)
Short fiction, Off the Main Sequence (2005)

“It is true that most religions which have been offered me teach immortality, but note the fashion in which they teach it. The surest way to lie convincingly is to tell the truth unconvincingly. They did not wish me to believe.”

Robert A. Heinlein book They

They (p. 60)
Short fiction, Off the Main Sequence (2005)

“Why do you like to play chess so well?”

Robert A. Heinlein book They

“Because it is the only thing in the world where I can see all the factors and understand all the rules.”
They (p. 55)
Short fiction, Off the Main Sequence (2005)

“Rotation through a fourth dimension can’t affect a three-dimensional figure any more than you can shake letters off a printed page.”

Robert A. Heinlein

And He Built a Crooked House (p. 33)
Short fiction, Off the Main Sequence (2005)

“Yes, maybe it’s just one colossal big joke with no point to it.”

Robert A. Heinlein book Methuselah's Children

Lazarus stood up and stretched and scratched his ribs. “But I can tell you this, Andy, whatever the answers are, here’s one monkey that’s going to keep on climbing, and looking around him to see what he can see, as long as the tree holds out.”
Methuselah’s Children (p. 667; closing words)
Short fiction, The Past Through Tomorrow (1967)

“What course of action do you favor?”

Robert A. Heinlein book Methuselah's Children

“Me? Why, none. Mary, if there is any one thing I have learned in the past couple of centuries, it’s this: These things pass. Wars and depressions and Prophets and Covenants—they pass. The trick is to stay alive through them.”
Methuselah’s Children (p. 539)
Short fiction, The Past Through Tomorrow (1967)

“You have to believe evidence when you have it in front of you, or else the universe is just too fantastic.”

Robert A. Heinlein book Time for the Stars

Source: Time for the Stars (1956), Chapter 12, “Tau Ceti” (p. 122)

“Attend me. How do you prove that there are eggs in a bird’s nest? Don’t strain your gray matter: go climb the tree and find out. There is no other way.”

Robert A. Heinlein book Time for the Stars

Source: Time for the Stars (1956), Chapter 8, “Relativity” (p. 82)

“Learning isn’t a means to an end; it is an end in itself.”

Robert A. Heinlein book Time for the Stars

Source: Time for the Stars (1956), Chapter 7, “19,900 Ways” (p. 70)

“I decided not to cross any bridges I had burned behind me.”

Robert A. Heinlein book Time for the Stars

Source: Time for the Stars (1956), Chapter 7, “19,900 Ways” (p. 69)