"In Hinduism, and in particular Jnana Yoga and Advaita Vedanta, neti neti is a chant or mantra, meaning "not this, not this", or "neither this, nor that".
Adi Shankara was one of the foremost Advaita philosophers who advocated the neti-neti approach.
The Upanishads talk of a supreme existence, an impersonal oversoul, called Brahman, which goes beyond western-definitions of a personal God. Brahman by definition, encompasses all reality, and is therefore not completely describable. Neti-neti is therefore held as the approach to understand the concept of Brahman without using affirmative (and thereby inadequate) definitions or descriptions of Brahman.
The purpose of the exercise is to negate conscious rationalizations, and other distractions from the purpose of a meditation. It is also a sage view on the nature of the Divine, and especially on the attempts to capture and describe the essence of God. In this respect, the phrase succinctly expresses the standpoint of negative theology."
The yoga practice of neti neti, purifies the awareness which has become polluted by worldly distractions. It is a gradual process of realising the Self by considering what it is not: 'I am not this body, this mind, these emotions or sensations'. But when the Self is realised, everything one sees, all that one had denied, is suddenly recognised as the Self. The trees are the Self, the sky is the Self, this body is the Self, even thoughts are the Self. So the process is reversed. It becomes a case of 'this, this'; but rather than the Self-awareness being polluted and weakened by the world, everything flows naturally from the pure source of Self and is purified (or rather it is recognised in it's real nature, which was always pure, was never polluted); senses, emotions, thoughts, the body the milieu, all become saturated with, and compliant to, the pure desire of the Self. This is the paradox of yoga - by excluding the world one becomes it.
Ascending the mountain, distractions gradually disappear. First, human habitation becomes sparser, then vegetation thins towards the treeline. Eventually even rocks disappear, covered by a featureless blanket of snow near the peak.
But in the colourless, featureless world of the summit the climber discovers a spring fed by that same pure, featureless snow - The spring is the source of many rivers that give rise to life further down the slopes, and further still, on to the plains where bustling cities develop around them.
The ascent to Selfhood is similar. First one excludes all that appears to be the non-Self but ultimately realises that the Self is all-inclusive.
"He who sees not the Self in all,
sees not the Self at all."
-After Sikh wisdom