Pain is not terribly well understood. I mean, we generally know how nerve impulses convey a signal to the CNS, but how the brain chooses to interpret those signals as "pain" is still something of a mystery. There is an area of research called neuromatrix theory that started out trying to explain phantom limbs, but has possible applications to chronic pain as well. From the little I've read about it, the idea is that the brain can start interpreting normal sensations as painful, or can create sensations that are not there. While this is an actual area if research, unlike Sarno's books, there are some interesting similarities.