You must perceive your essential nature before you attain enlightenment. What is perceiving essential nature? It means perceiving your own original nature. What is its form? When you perceive your own origianl nature, there is no concrete object to see. This is hard to believe in, but all buddhas achieve it. 

                                                                              - Hsueh-feng (822-908)

I have become a big fan of these questions. They are 1100 years old and can still stop me right in my tracks. I think I have an affection for answers, my comfort and self esteem respond very well to correct answers. Getting acknowledged for a good answer by someone else is icing on the cake. What is perceiving essential nature does not really allow for a quick answer. It invites me to come and sit for moment, quiet the ongoing monologue in my head and pay attention to it's essence.