what was mr.murry's explanation as to how he could keep from being absorbed by IT

On page 158, Mr. Murray says, "Because IT's completely unused to being refused. That's the only reason I could keep from being absorbed too. No mind has tried to hold out againsy IT for so many thousands of centuries that certain centers have become soft and atrophied through lack of use. If you hadn't come when you did I'm not sure how much longer I would have lasted. I was on the point of giving in."