We build up a self-concept based on experiences, beliefs, opinions, sets of thoughts and emotions. And we cling to this concept because it creates what we think is a safe rational place. But unfortunately when something happens to shake the foundation of who we think we are (e.g. losing a loved one) our entire view of who we are (including our relationship to everyone around us) falls apart. So we panic and fight to keep it together. Instead of trying to make everything fit in our comfort zone, we should instead be questioning the model we've built in the first place.
A very good friend of mine follows this philosophy -- believe in nothing, because dogma is dangerous. You will never by happy if your identity is tied to form or beliefs because beliefs should evolve with new information. This is wise. This book says that you should be free to experience life without mental boundaries. This 'going beyond' is enlightenment.