“The trouble about man is twofold. He cannot learn truths which are too complicated; he forgets truths which are too simple.”

Source: Lover Reborn

"The Trouble with Man is Man", The New Yorker; reprinted in Lanterns & Lances (1961).
From Lanterns and Lances‎

"The Mathematician", in The Works of the Mind (1947) edited by R. B. Heywood, University of Chicago Press, Chicago
Context: I think that it is a relatively good approximation to truth — which is much too complicated to allow anything but approximations — that mathematical ideas originate in empirics. But, once they are conceived, the subject begins to live a peculiar life of its own and is … governed by almost entirely aesthetical motivations. In other words, at a great distance from its empirical source, or after much "abstract" inbreeding, a mathematical subject is in danger of degeneration. Whenever this stage is reached the only remedy seems to me to be the rejuvenating return to the source: the reinjection of more or less directly empirical ideas.

Light (1919), Ch. XVI - De Profundis Clamavi

Listen, Little Man! (1948)
Context: My intellect tells me: "Tell the truth at any cost." The Little Man in me says: "It is stupid to expose oneself to the little man, to put oneself at his mercy. The Little Man does not want to hear the truth about himself. He does not want the great responsibility which is his. He wants to remain a Little Man. He wants to remain a Little Man, or wants to become a little great man. He wants to become rich, or a party leader, or commander of a legion, or secretary of the society for the abolition of vice. But he does not want to assume responsibility for his work..."

The Origins Of History (1981)

Source: The Amber Spyglass

Letter to Lady Elizabeth Pelham (4 January 1939))

2:2 <!-- p. 317 -->
Paraphrased variant: Man can certainly flee from God... but he cannot escape him. He can certainly hate God and be hateful to God … but he cannot change into its opposite the eternal love of God which triumphs even in his hate.
Quoted in Simpson's Contemporary Quotations (1998) by James Beasley Simpson.
Church Dogmatics (1932–1968)
Context: Man can certainly keep on lying (and he does so); but he cannot make truth falsehood. He can certainly rebel (he does so); but he can accomplish nothing which abolishes the choice of God. He can certainly flee from God (he does so); but he cannot escape Him. He can certainly hate God and be hateful to God (he does and is so); but he cannot change into its opposite the eternal love of God which triumphs even in His hate. He can certainly give himself to isolation (he does so — he thinks, wills and behaves godlessly, and is godless); but even in his isolation he must demonstrate that which he wishes to controvert — the impossibility of playing the "individual" over against God. He may let go of God, but God does not let go of him.