Sentence:  Brittany , that stone-gray mystery through which he traveled for thirty days , sleeping in the barns of farmers or alongside roads , had worked some subtle change in him , he <WSD>knew</WSD> , and it was in Brittany that he had met Pierre . 
Meaning of the word marked with <WSD> tags:  be aware of the truth of something; have a belief or faith in something; regard as true beyond any doubt