I read the book around 1964 in French. I cannot remember any thing but 1/ it was set in the Spanish Civil War, 2/ a story right at the first pages as fallows:
A young guy, probably the main character in further development, wished to join the Resistance in deep jungle. He was discouraged by people who told him about feral animals, snakes...government police... But he takes the adventure successfully. Then he got the idea that people just talked about what they didn't really encounter.
Later the guy was arrested, tortured and broke down, revealed his cell members' identities. The same people laughed at him, made fun with his "cowardice". He told himself, they've never been tortured to know that because of the torture one is no more one, unable to control the mind. In both case, they just talked, without live experience.
In the real life, Arthur Koestler was tortured by Gestapo but he didn't break down.