I am consciousness brahman

We have examined what happens to the physical and subtle body of an ignorant person. What about his ātmā? Does it travel like the subtle body? There is only one ātmā which is all-pervading; ātmā does not differ between a wise person and an ignorant person. Therefore, there is no possibility for ātmā to travel. How about the causal body? When ignorance of the oneness is present, the causal body (ahankara) is there, therefore it is the cause for the other two bodies to exist. When ignorance of oneness is removed, there is no cause for the other two bodies to reborn, thus causal body is resolved.

Mantra 8 through 15 are the central teaching of this section, because they discuss the central theme of the Upaniṣad: the nature of brahman. In the next mantra, yamaraja reveals that brahman is the very consciousness self alone.

ya eṣa supteṣu jāgarti kāmam kāmam puruṣo nirmimāṇaḥ |

tadeva śukram tad brahma tadevāmṛtamucyate |

tasmimlokāḥ śritāḥ sarve tadu nātyeti kaścana | etadvai tat ||2-2-8||

This puruṣaḥ keeps awake projecting various (dream) objects when (all senses are) asleep. That is indeed said to be brahman, who is pure and immortal. All the worlds are based on that. Verily, nothing exceeds that. This is indeed that.

puruṣaḥ - the indweller of the body, the very self is ya eṣa supteṣu jāgarti - awake (illumines everything all the time) even when sense organs are asleep. The sense organs illumine the world when they are awake and do not illumine when they are asleep. Therefore, the sleeping state is mentioned here to point out that sense organs (including the mind) are merely temporary illuminator in the waking state. The real illuminator is the puruṣaḥ (consciousness) alone. When the mind is active, the consciousness illumines the active mind and when the mind is asleep, consciousness also illumines the sleeping (condition of) mind. When I wake up from deep sleep, I say “I did not know anything in sleep”. How do I know that I did not know anything in sleep? Because I am the illuminator of the knowing and the non-knowing mind. I know that my mind know. I also know that my mind doesn’t know. I am the ever-effulgent consciousness.

In what way this jīva ātmā keeps awake in the dream state? kāmam kāmam nirmimāṇaḥ - projecting various desired objects, in the form of dream. Since avidyā has veiling power, the dreamer status is covered during the dream state, thus one doesn’t know that one is dreaming and projecting the dream world from the impressions gathered through his mind. tadeva śukram tad brahma - that ātmā alone (who is awake in waking, dream and deep sleep state) is brahman which is pure (not identified with the mind) and bright (illumines all). tadevāmṛtamucyate - that brahman alone is called immortal, not subject to death, limitless in terms of time and space. It is moksa - free from boundaries.

Why is it free from boundaries? Because tasmimlokāḥ śritāḥ sarve - all worlds (spheres of experience like earth, heaven, etc.) are dependent upon that brahman for existence and evident, because brahman is the cause of all. tadu nātyeti kaścana - indeed nothing goes beyond brahman. This consciousness - me is that brahman who is the basis of all, this is what naciketa wanted to know -  etadvai tat.