thinking in the letter vs the spirit

Somewhat scary that the physical body can overtake the spirit so easily and bring one's concentration back into a fine point upon itself. To what end? To what end does pain drag the concentration so quickly back into the temporary body? Am I on the right track here?

Had lunch one particular time, lost in memory now, but upon leaving it suddenly thrust itself into my realization that he was talking the entire time in the letter and I was talking in the spirit. This is the way it worked out. I mentioned that C.S. Lewis had a crisis in faith when his wife died. Faith and the achieved strength of it is often my primary concentration. But I quoted Lewis as saying it was all a fine kettle of fish. 

Whereupon my companion exclaimed Oh Spence I doubt if he said that. My first reaction was, yes he said exactly that. Then it hit me. He was in the letter all the time while I'd been talking in the spirit. So what did Lewis say then? Was it a fine can of worms, which is the same thing? What did he say? I have no idea what he said letter-wise, but the spirit of the matter was that the entire situation was a fine kettle of fish which captured the spirit of the issue quite precisely. Probably pick this up eventually in Memorist.