‘I come’ he said ‘not to destroy the Dharma, but to reclaim it’.
By the time of the arrival of Siddhartha Gautama, the literature on That had gotten ponderous, opaque and layered in mystique.
That [about which nothing may be said or elaborated] soon gets elaborated into an: ‘Inner Essence’. And this ‘Inner Essence’ soon takes on a life of its own and is termed ‘Thou’. [‘Self’; See: ‘The ‘Inward Turn’]
In time this establishes itself as: Tat Tvam Asi: ‘That Are’t Thou’. The Summum Bonum of the Dharma itself.
A ‘That’ and a ‘Thou’, both terms left dangerously undefined, layered in mystical cant, that take the Original Dharma wildly off-course.
Siddhartha Gautama’s chosen name for himself was not as ‘The Buddha’ [a later appellation] but as the Tat-āgathā, the same ‘Tat‘ of the Rig Veda.
Wherefore the distinction? ‘Tat‘ is That; Tat-āgatha is: One ‘Entered in That’ [or ‘That-Gone’].
And ‘That-Gone’ has nothing at all to say about any Object. It is exclusively about the Subject. Or rather, the absence of it.
Tat-āgathā [or ‘That-Gone’] is the fulfillment of Tat Tvam Asi: ‘That Are’t Thou’.
At the final terminus of Shūnyam, the need for a reference to That itself dissolves.
In the highest tradition of the Buddha-Dharma there is no intermediate step. One does not worship The Buddha, which is easy enough. The struggle is to become a Buddha; a Tat-āgatha.
The Diamond Sūtra uses Nirvāṇa and Tat-āgatha alternatively and synonymously. They represent the last use of conventional language to communicate the Logical Form of the Self-Eating Expression. Tat to Tat-āgatha. And Tat-āgatha to Shūnyam.
If one violates the Inexpressibility of That, the least one must do is to define and make explicit these violations. To nurture them instead as themselves ‘inexpressible insights’ leaves one speechless. The sharp turn away from the terminus was inevitable.
The primal expression Tat morphs its meaning twice. First as Tathātā or ‘That-ness’, an abstract extension primarily to separate philosophical content from grammatical use. And secondly as Tat-āgatha. [I have seen commentaries where these get conflated, a grievous error.]
See: ‘Rounding Yagnavalkya’s Rule‘