“It ain't the parts of the Bible that I can't understand that bother me, it's the parts that I do understand.”

American Rust (2009)

Source: Bag of Bones

Source: Always True (Moody, 2011), p. 91

Interview, quoted in "Words from the Master" http://www.co.uk.lspace.org/books/apf/words-from-the-master.html in The Annotated Pratchett File http://www.co.uk.lspace.org/books/apf/index.html
General sources
Context: As for The Mapp... I suspect it'll never get a US publication. It seemed to frighten US publishers. They don't seem to understand it.
That seems to point up a significant difference between Europeans and Americans:
A European says: I can't understand this, what's wrong with me? An American says: I can't understand this, what's wrong with him?
I make no suggestion that one side or the other is right, but observation over many years leads me to believe it is true.

dig interview (2004)
Context: I don't feel any form of music is beyond me in the sense of that I don't understand it or I don't have some love for some part of it. And if there's something in it that I can respond to, then there's something that I might be able to use as a composer. There are records of mine that have had smaller audiences and have provoked really drastic responses from people — particularly from critics — who maybe don't have quite enough time to live with the record or accept that a piece has its own integrity.

ContraPoints, Miscellaneous, Contrapoints Is De-Radicalizing Young, Right-Wing Men https://www.youtube.com/watch?v=2Nrz4-FZx6k (Vice News, 2019)

Source: Romancing Mister Bridgerton

Attributed to Einstein by physicist John Archibald Wheeler in John Horgan's article "Profile: Physicist John A. Wheeler, Questioning the 'It from Bit'". Scientific American, pp. 36-37, June 1991. Reprinted here http://www.scientificamerican.com/article.cfm?id=pioneering-physicist-john-wheeler-dies after Wheeler's death.
Attributed in posthumous publications