Sri Aurobindo translates Katha Upanishad, Second Cycle: Third Chapter, Verses 12-13: “Not with the mind has man the power to get God, no, nor through speech, nor by the eye. Unless one says ‘He is’, how can one become sensible of Him? One must apprehend God in the Concept ‘He is’ and also in His essential: but when he has grasped Him as the ‘Is’, then the essential of God dawns upon a man.”
The Upanishads repeatedly remind us that God cannot be apprehended by the senses, through words, or the action of the mind. All of these are powers of the external world, and by nature see and categorize everything in a fragmented manner as separate forms, beings, forces and events. It is easy to find focus on all the differences and pay attention to the interaction of these separate entities when one bases one’s perception and understanding on these outer tools of knowing and acting.
It takes a different type of understanding, a different perception to see and know God. It is not through a focus on the outer forms that we recognize the reality of the Divine Spirit. That is why the Upanishads remind us of “neti, neti” (not this, not that) so that we do not get lost in the forms and miss the “forest for the trees” as we might see in our modern language.
It is just this sense of the “forest” consisting of innumerable trees of different size, shape and type, that provides us an example for true understanding. The trees do not exist without a cause, without a purpose and without a basis. God is that cause, God is that purpose, God is that basis! When we shift our view away from the fragmented view of the mind, the senses and the linear expressions of speech, toward the unifying factor that pervades, contains, creates, and is the constituent basis of all that exists, then only can we perceive God as all, in all and beyond each and every specific manifestation.
All this is the Brahman, but if we try to grasp any particular thing, we limit the Divine. One without a second is true, and does not thereby negate the existence of the innumerable forms and forces we see around us. When we understand that the mental power is unable to encompass God’s reality, then we can begin to experience the “the Truth, the Right, the Vast” that is God’s reality.
Sri Aurobindo, The Upanishads, Katha Upanishad, pp. 213-241, and Kapali Sastry, Lights on the Upanishads, pp. 104-129