Widest means it includes everything. After all, whatever is is existing. So when you ask, what is existence? Then every liquid thing is included. One might say, just a minute, non existence is not included in existence. But non existence does not exist. And there have been actually books written on shankara and hideger. For them, they were seriously dered. They ere seriously interested in the question of non existence. We won't go into that.
Inspired by the 15th chapter of the Ashtavakra Gita, Swami Sarvapriyananda gives a discourse on the nature of the Self.