Having distinguished between the higher and the lower knowledge in Chapter 1, the Mundaka Upanishad focuses its attention on the nature of the higher knowledge, and its means of attainment in Chapter 2. At the same time, the apparent duality described at the close of Chapter 1 is taken up and clarified here.
Sri Aurobindo translates Chapter 2, Section 1, Verses 1-2: “This is That, the Truth of things: as from one high-kindled fire thousands of different sparks are born and all have the same form of fire, so, O fair son, from the immutable manifold becomings are born and even into that they depart. He, the divine, the formless Spirit, even He is the outward and the inward and He the Unborn; He is beyond life, beyond mind, luminous, Supreme beyond the immutable.”
The outer world and all its forms and beings are shown here to originate in the immutable and return thereto after death. The question raised here is one that appears in several Upanishads, namely, from whence are all things born, what sustains them in life, and whereto do they depart upon death. This is a seeking after “first cause” which a focus solely on the powers, forms and forces of the world misses. Western scientists, for instance, stop at the “big bang” but do not tend to look further back into Time, before the “big bang” to understand from whence it arose. As they expand their vision into this realm, they will take up the ultimate question of existence with which the Upanishadic sages grappled long ago.
Verse 2 gives us a hint of what became, in the Bhagavad Gita, a well-defined concept, namely that of the Purushottama, the divine Being beyond both change and immutability, beyond Kshara Purusha and Akshara Purusha, “Supreme beyond the immutable.” This also begins the integration of the two great concepts “One without a second” which, on its own led to the exclusive focus on the higher knowledge, and “All this is the Brahman.” which reminds us that the entire manifested universe, the focus of the lower knowledge, is none other than the Brahman.
The Supreme transcends all the forms, forces and beings in the universe, contains, yet is not limited by them. The Supreme also is shown here to be beyond the Immutable. When we try to grasp the Supreme through mind or speech, it is found to be impossible, as it always transcends the widest scope our minds can provide for it.
Sri Aurobindo, The Upanishads, Mundaka Upanishad, pp. 193-210