“Not this, not that” proclaim the Rishis of old when they tried to describe the Absolute Brahman, beyond the reach of the mind and the senses. The intention here is not to frame the Absolute as something “negative” but to ensure that we do not believe we know it in any true sense by the powers of the mind, which are limited to the frame of the manifested universe. The vastness of the manifestation and the unity of the creation make it even impossible to know all of the universe through our limited mental powers. How much more impossible, then, it will seem, if we try to encompass also the silent Absolute beyond all manifestation with a power that is limited, fragmented and distorted in its view and standpoint?
The Second Part, verse 2 adds to the review of the process of knowing: “I think not that I know It well and yet I know that It is not unknown to me. He of us who knows It, knows That; he knows that It is not unknown to him.”
Sri Aurobindo comments: “[The Upanishad’s] answer to the problem is that That is precisely the Unknowable of which no relations can be affirmed and about which therefore our intellect must for ever be silent. The injunction to know the utterly Unknowable would be without any sense or practical meaning. Not that That is a Nihil, a pure Negative, but it cannot either be described by any of the positives of which our mind, speech or perception is capable, nor even can it be indicated by any of them. It is only a little that we know; it is only in the terms of the little that we can put the mental forms of our knowledge. Even when we go beyond to the real form of the Brahman which is not this universe, we can only indicate, we cannot really describe. If then we think we have known it perfectly, we betray our ignorance; we show that we know very little indeed, not even the little that we can put into the forms of our knowledge. For the universe seen as our mind sees it is the little, the divided, the parcelling out of existence and consciousness in which we know and express things by fragments, and we can never really cage in our intellectual and verbal fictions that infinite totality. Yet is is through the principles manifested in the universe that we have to arrive at That, through the life, through the mind and through that highest mental knowledge which grasps at the fundamental Ideas that are like doors concealing behind them the Brahman and yet seeming to reveal Him.”
Sri Aurobindo, The Upanishads, Kena Upanishad and analysis, pp. 103-104, 165-170