The main figure in the Upanishads is sage Yajnavalkya, known as one of the greatest philosophers. Most of the great teachings of later Hindu or Buddhist philosophy are derived from him. He taught the great doctrine of neti–neti, the view that truth can be found only through the negation of all thoughts about it.

Brhadaranyaka Upanishad is the oldest and the most important of all the Upanishads. Its name actually means the great forest book.

Sage Yajnavalkya’s dialogues with his wife, Maitreyi are featured in the Muni Kanda or Yajnavalkya Kanda. The doctrine of neti–neti suggests the indescribability of the Brahman, the Absolute. Yajnavalkya attempts to define Brahman.

Atman is described "neither this, nor this" neti–neti. The Self cannot be described in any way. Na–iti – that is Neti. Through this process of neti–neti you give up everything - the cosmos, the body, the mind and everything – to realize the Self.

Once the Atman is defined in this manner and you become familiar with it, a transformation takes place as realization dawns that the phenomenal world and all its creatures are made up of the same essence of bliss.

Brahman is infinite, amorphous, colorless, characterless and formless Universal Spirit, which is omnipresent and omnipotent, and like cosmic energy, is pervasive, unseen and indescribable.

• Neti–neti Meditation: The principle of neti–neti has been used in meditation involving gnana yoga. Whenever a thought or feeling comes to mind that is not the goal of the meditation, or is not the soul or the inner self, the meditator simply has to say, "Not this, not this," and dismiss the thought, image, concept, sound, or sense distraction. Any thought, any feeling, is patiently discarded — again and again if necessary, until the mind is clear and the soul/or the self is revealed.
• Neti–neti and the mind: When you get into the habit of neti–neti, you can also discard thoughts of worry, doubt, or fear, and become established in the light of your inner self. You can, then, look back at worries and fears with deep insight and handle them.
• Neti–neti and the medical profession: One of the basic medical teachings is to diagnose a condition by way of excluding other similar conditions. This is called differential diagnosis and this is the mainstay of allopathy. This also makes one investigative-oriented but is the only scientific way of knowing the truth.
• Neti–neti and multiple–choice questions: In any modern exam today, the principle of neti–neti is used. A question has about four nearly similar answers and the student has to answer the correct one. He can only answer by the principle of negation.
• Neti–neti and police investigations: This principle is also used while handling a criminal case. Everyone is a suspect in the crime initially, till a process of elimination clears them.