You may not say a word about ‘That‘. And any word you say about ‘That’, by that very fact is not ‘That’.
I cannot express ‘The Inexpressible’ [even as the ‘Inexpressible in counterpoint to the ‘Expressible’]. Anything I express as the ‘Inexpressible’, by that very fact is not ‘The Inexpressible’.
The track to ‘That’ is identical to the ‘Backward Step’ that generates from the ‘Axioms of Sight’. And as with the axioms, effecting this yields the Logical Form of the Self-Negating Expression.
The Symbol ‘0’ [Shūnyam] is simply the Logical Form of ‘That’ taken to its natural, necessary and inevitable limit. And unlike with ‘That’, long cloaked in an occult opacity, Shūnyam is explicit, irreducible, achievable and verifiable. You cannot fudge it.
The expression ‘That‘ preceded the construction of Shūnyam by likely a millennia. Shūnyam was designed as a guiding-rail. Without it the ‘Backward Step’ is not navigable. You will spin indefinitely in self-referential loops with no exit.
You either sight Shūnyam or you don’t. Like the Nerds say: ‘It’s a ‘0,1’ thing’.
In the centuries following, the inevitable cycle once again reasserts itself. The intuition of Shūnyam gets morphed into a rarefied space of high-abstractions and elevated reifications, all proxying for a missed denouement. [The Posts list about 40 examples.]
There is a literature in the more sophisticated inquiries that equates ‘Nothing’ [‘Creatio Ex-Nihilo‘, and similar proposals] with variations of a very Man-Made ‘Creator God’. That track is well explored and won’t get you far. Stay with Shūnyam.