Neti neti appears in the Bṛhadāraṇyaka Upaniṣad as the teaching about the self: it is not this, not this, for nothing can grasp it. Every object of perception or thought is set aside as not the self. The method became central to Advaita Vedānta and is often compared with the apophatic way of Christian and Neoplatonic mysticism.
Neti neti
“Not this, not this”: the Upaniṣadic way of pointing to Brahman by denying every description of it.
Also: Neti, neti