This is the passage which foreshadows his The Two Cultures twenty-seven years later.
"You'll soon be sure that I'm not a murderer," I chaffed him. He laughed, and lit a cigarette. His glance was faraway as he spoke:
"I just wanted to see what I could do with the facts when I proved you were innocent. People talk about material truth and psychological truth as though, if you are interested in the one you can't be interested in the other. Of course that's nonsense. If I had all the material facts, I shouldn't want any psychological facts. That is, if twenty-eight different and reliable witnesses had seen Roger shoot himself before their eyes, then I should believe it, though it's psychologically improbable the anyone like Roger would shoot himself. In exactly the same way, if I knew all the psychological facts about – say, Aloysius Birrell – and they made it quite certain that he was bound to kill Roger, then I should believe that, though it seems to me materially improbable. But the point is, one never has all the material facts or all the psychological facts. One has to do what one can with an incomplete mixture of the two.
Fibow smiled: "Yes, I must tell Aloysius Birrell that. He carries a notebook in which he writes down all the great thoughts on criminology. Anyway, all that I can say from the material facts so far is that any one of five people may have killed Roger. Any fool could have arrived at that. The important thing is to find some of the psychological facts."