The central summary expression of Vedic insight is Tát, literally ‘That’.
‘That’ is an Expression of Inexpressibility. A self-scuttling assertion in negation, an immediate, unregenerate self-contradiction. It is neither noun nor verb, is grammatically homeless, a lexicographer’s nightmare, and meant to be so.
Point a finger, draw a line, say a word, think a thought, emote a feeling towards ‘That’ and by that very act, what you have pointed a finger to, drawn a line of, said, thought, emoted, is not ‘That’.
It includes all markers, any and every whiff of presumption to identity. Aspects, elements, endowments, features, qualities, temperament, tendencies. You may not source it for ethical or social directives [rules], go philosophical or poetic on its attributes…
The Summum Bonum of Dharmic Teaching is: Tát Tvam Asi, the same Tát [‘That’] of the Rig Veda. In translation: ‘That are’t Thou’.
I cannot say anything about ‘That’. Anything I say about ‘That’, by that very fact is not ‘That’. Effecting this yields the Logical Form of the Self-Negating Expression.
The Symbol ‘0’, Shūnyam is simply ‘That’ taken to its natural, necessary and inevitable limit.