Sri Aurobindo translates Mundaka Upanishad, Chapter 2, Section 2, Verses 1-2 as follows: “Manifested, it is here set close within, moving in the secret heart, this is the mighty foundation and into it is consigned all that moves and breathes and sees. This that is that great foundation here, know, as the Is and Is-not, the supremely desirable, greatest and the Most High, beyond the knowledge of creatures. That which is the Luminous, that which is smaller than the atoms, that in which are set the worlds and their peoples, That is This, — it is Brahman immutable: life is That, it is speech and mind. That is This, the True and Real, it is That which is immortal: it is into That that thou must pierce, O fair son, into That penetrate.”
Wise men, seers, Rishis throughout the ages, have struggled with the way to describe the indescribable, to define the indefinable. The Brahman cannot be limited by any description we can provide. If we try to provide a positive description, we find that our language cannot go far enough to encompass and embrace the reality of the Brahman. Thus, wherever there is a positive description, there is also a “negative” description which reminds us that whatever ideas we have, whatever ways we have to formulate the nature of the Brahman into thoughts, into words, into speech, they are never going to be enough. The Brahman is “not this, not that” because it exceeds every limitation and description.
In the present verses, the Mundaka Upanishad provides as good a description as anyone by implying that all that exists is the Brahman, while concurrently making sure that we appreciate that it always goes beyond any of our descriptions. It is unknowable, and yet, the goal of the seeker is to know the Brahman. How is this possible? While it is beyond mind, beyond speech, beyond description with words, the Brahman is also in our secret heart. We are not separate from the Brahman, so while we cannot define the Brahman through logic, reasoning, intellect or with the use of words, we can know the Brahman through a knowledge by identity.
Sri Aurobindo, The Upanishads, Mundaka Upanishad, pp. 193-210