It's from C.P. Snow's The Light and the Dark (1948), making the point that self-knowledge, stripped of arrogance is crucial to those who hold power. Here's Snow's stand-in Lewis Eliot, debating the balance of power in 1937 with a young Nazi:
"No one is fit to be trusted with power," I said..."No one. I should not like to see any group of men in charge--not me or my friends or anyone else. Any man who has lived at all knows the follies and wickedness he's capable of. If he does not know it, he is not fit to govern others. And if he does know it, he knows also that neither he nor any man ought to be allowed to decide a single human fate, I am not speaking of you specially, you understand; I should say exactly the same of myself."The Light and the Dark (first ed.), at pp. 148-149.
Our eyes met. I was certain, as one can be certain in a duel across the table, that for the first time he took me seriously.
"You do not think highly of men, Mr. Eliot."
"I am one."
I buy that. I believe that, even today.