@igm,
You have uncovered a 'flaw' in Guigus' thinking. Guigus is saying that no matter how hard he tries he can't prove that he (pure 'Be'-ing) exists.
He is saying that he would rather hide behind what he can prove instead of taking a leap-of-faith and stand on his own 2 feet.
Authenticity is a rigorous moment-to-moment choice you make. 99.999999% of the people in the world are entangled in the provability (definability and measurability) of the labrinth we call the world and don't know that who they are and the world they live alongside are 2 seperate entities.
If you spend your life on the planet using the world's criteria of length, width, depth, mass, and locality in an attempt to prove your existence you will think you are a 'thing' of this world and never know who you are.
It is hilarious to note that members of the 'Philosophy Forum' are using philosophy to prove to the world that they can hide out for a lifetime.