“There's nothing wrong with not understanding yourself.”

Source: Grass (1989), Chapter 7 (pp. 140-141)

Source: City of Heavenly Fire

1850s, Speech at Peoria, Illinois (1854)
Context: The foregoing history may not be precisely accurate in every particular; but I am sure it is sufficiently so, for all the uses I shall attempt to make of it, and in it, we have before us, the chief material enabling us to correctly judge whether the repeal of the Missouri Compromise is right or wrong.
I think, and shall try to show, that it is wrong; wrong in its direct effect, letting slavery into Kansas and Nebraska — and wrong in its prospective principle, allowing it to spread to every other part of the wide world, where men can be found inclined to take it.
This declared indifference, but as I must think, covert real zeal for the spread of slavery, I can not but hate. I hate it because of the monstrous injustice of slavery itself. I hate it because it deprives our republican example of its just influence in the world — enables the enemies of free institutions, with plausibility, to taunt us as hypocrites — causes the real friends of freedom to doubt our sincerity, and especially because it forces so many really good men amongst ourselves into an open war with the very fundamental principles of civil liberty — criticising the Declaration of Independence, and insisting that there is no right principle of action but self-interest.

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.

1960s, Freedom From The Known (1969)
Context: That is the first thing to learn — not to seek. When you seek you are really only window-shopping. The question of whether or not there is a God or truth or reality, or whatever you like to call it, can never be answered by books, by priests, philosophers or saviours. Nobody and nothing can answer the question but you yourself and that is why you must know yourself. Immaturity lies only in total ignorance of self. To understand yourself is the beginning of wisdom.

Source: Goldilocks and the Three Dinosaurs