The most fundamental questions are which also include themselves. This is called self reference and leads to interesting paradoxes in inlogic. When you ask, what is existence, you are asking about that question itself. O this is a self referential question also. So hidraker says, it is the widest question, deepest question and the most the fundamental question. What is existence? Isness? I have earlier mentioned that book, very nice book, jim holt, why does the world exist? Very beautiful book.
Inspired by the 15th chapter of the Ashtavakra Gita, Swami Sarvapriyananda gives a discourse on the nature of the Self.