“Men pass in front of our eyes like butterflies, creatures of a brief season. We love them; they are brave, proud, beautiful, and clever.”

On Publishing
Writers at Work (1977)
Context: A book is like a man — clever and dull, brave and cowardly, beautiful and ugly. For every flowering thought there will be a page like a wet and mangy mongrel, and for every looping flight a tap on the wing and a reminder that wax cannot hold the feathers firm too near the sun.

The origin of this quote is often misattributed to Cicero; however, it is from Line 135-136 of Book 2, Satire 2 by Horace, "Quocirca vivite fortes, fortiaque adversis opponite pectora rebus." The English translation that most closely matches the one misrepresented as Cicero's is from a collection of Horace's prose written by E. C. Wickham, "So live, my boys, as brave men; and if fortune is adverse, front its blows with brave hearts."
Misattributed

Book II, Satire II, Line 135-136 (trans. E. C. Wickham)
Satires (c. 35 BC and 30 BC)

Interview with the Hollywood Reporter (4 February 2020)

Address at Iona College (1984)

Source: Did You Miss Me?

"We Call Them the Brave" (the title of this poem is also obviously meant to be read as its first line, though set apart)
The Poems of Marianne Moore (2003)

Source: The Merry Adventures of Robin Hood