Lets wander a bit, East and West of ‘That’. I think it was Hakuin who wrote that all of Zen was contained in Mu.
‘That’ [Tát] as an ‘Expression of Inexpressibility’ is the pivotal expression in the Dharmic Tradition. But there is no serious tradition that does not carry it, or some close variant of it.
From the opening line of the opening verse of the Tao Te Ching:
‘The True Tao is nameless; what is named is not the True Tao. The True Tao cannot be told; what is told is not the True Tao’.
Tao is a Self-Negating Expression.
The central directive of Taoism is to live a life based on Wie Wu Wei: ‘Doing Not-Doing’. A Self-Negating Expression. Of Course.
Lao Tzu [circa 5th Century BCE] founded Taoism, the first philosophy of China. Lao Tzu didn’t prissy around. First he declares that the true Tao is Nameless. He then adds that nothing may be told about it. All in the opening verse.
He then proceeds to write his poem, the seminal Tao Te Ching, naming and telling all about the Tao. [Sort of like this Site.]
The opening Koan from the venerated Mumonkon Collection is: ‘Joshu’s Mu!’. It is in response to the question: ‘Does a dog have Buddha-Nature?’.
Joshu’s ‘Mu!’ is a negative particle, a vociferous assertion of negation. The driving kinetic of its terse formulation is its simultaneous self-consumption. Literally, ‘Nothing’.
A fiery, full-blast Self-Negating Expression.
Bodhidharman defined the ends of C’han-Zen Practice as follows: ‘A special transmission outside the scriptures; no dependence on words and letters; seeing into one’s Self-Nature, and the attaining of Buddha-hood.’
A more prosaic way of simply saying ‘That’ in pursuit of pedagogic propriety.
Again from the Mumonkan:
Daibai asked Baso: ‘What is the Buddha?’
Baso answered: ‘The mind is the Buddha.’
A monk asked Baso: ‘What is the Buddha?’
Baso replied: ‘Not mind, not Buddha.’
Note that the better Koans never mark an identifiable terminus. It dates back to the founding of the Tradition itself.