Around 1,500 BCE, the oral tradition descends to the written word. And in the Bṛhadāraṇyaka Upanishad, Uddalaka’s distinguished disciple Yājñavalkya in a venerated dialogue with his wife Maitreyi lays out a simple rule to get to That.
The formula is: ‘It is not this! It is not this!’
If you can point to it, sense it, imagine it, name it, it is a ‘This’. More generally, if you can have any kind of relationship with something, the ‘something’ is a ‘This’.
A thousand years later, in the language of the Diamond Sūtra, the direct heir to Yagnavalkya’s Rule: ‘A Bodhisattva should develop a mind which alights upon no thing whatsoever; and so should he establish it‘.
This was the earliest [if incomplete] formulated definition of the Symbol ‘0’.
Yagnavalkya is the first documented Inquirer I know to have taken the trek away from imaginative and emotional constructs of the nature of the Divine into one of almost clinical abstraction. In his day and age, this was an assault on the Gods, a very risky business. But then this was still the region of India. See: Varanasi.