Neti Neti is the definition of Supreme Truth (Brahman) employed in Brihadaranyaka Upanishad. Rig Veda contains the germ of the idea of a single entity behind and beyond the transient phenomena of the universe. The great teachers of Sanatana Dharma (Vedic Rishis) were not satisfied with the anthropomorphic conception of several gods such as Indra, Varuna, Vayu and Agni. Through constant questioning and contemplation, they arrived at the view that the Reali is One, the learned called it by various names – Agni, Yama, and Matarisvan (ekam sad vipra bahudha vadanti, agnim yama,, matrasivanam ahuh) (I-164.46).
This unified conception of reality – “the eternal spirit ever acting and ever resting” – is better defined and clarified in Upanishads.
Upanishads posit Brahman as the Ultimate Reality. The nature of Brahman is defined as sat-chit-ananda – truth consciousness bliss.
But in explaining the precise relationship between the real and the phenomenal, the changeless and the changing, Upanishads are not unanimous but differ in their expositions, viz., Brahman as different from the universe, Brahman as transcendental and immanent and Brahman as something beyond description.
The indescribability is apparent when Brahman is sought to be compared with the known phenomena of the universe. Neti-neti (not this, not this) is the only answer that is possible when a comparison is made. If we say a cup is blue, the logically entails that the cup is not green, red, yellow, etc. If Brahman is all, the positive predication will delimit falsely the totality that is Brahman.
The great teachers of Upanishads recognized the dilemma and thus resorted to negation. Kena Upanishad defines Brahman as that which cannot be expressed in words but by which the words are uttered by the tongue, that which is not comprehended by the mind but by which the mind comprehends, etc. According to Mundaka Upanishad, it is the one “by knowing which everything else is known.”
In Brihadaranyaka Upanishad (Satapatha Brahmana of Shukla Yajurveda), the two terms “neti neti”occur, and Brahman is described as satyasya satyam (the truth of truth).
Brahman, the Supreme Being, according to Brihadaranyaka Upanishad has two forms. One is gross, limited and perceptible, perceived by the sense organs. All gross things are limited and can be defined. The essence of that which is gross, limited and defined is the sun that shines, for it is the essence of the defined. But that which is subtle, immortal, unlimited and cannot be defined is Brahman. Its name is “Truth of Truth.” It is to be experienced and, therefore, inexpressible.
Source –
The Spiritual Heritage of India (1979) Swami Prabhavananda – Vedanta Press South California.
Encyclopedia Of Hinduism Volume VII page 450 – 451 - IHRF