Neti-neti — Sanskrit for “not this, not this” — is the method by which the Upanishads and Advaita Vedanta point at what cannot be directly named. Every time the student identifies the self with something observable — body, mind, role, feeling — the teacher says: neti, neti. Not this. Not this either.
What remains when everything observable has been set aside as not the observer is Atman. This is the method underneath Ramana Maharshi‘s question who am I? and parallels exactly the Christian Apophatic tradition.
“This self is, therefore, described as ‘Not this, not that.’ It is unseizable, for it is not seized.” — Brihadaranyaka Upanishad 3.9.26