Cause & effect relationship

Based on the logic of the co-present, mantra 2.2.3 establishes: when ātmā (in the form of reflected consciousness) is present, the body is alive, and the logic of co-absence is explained in mantra 2.2.4: when the same ātmā is absent, the body is dead. Therefore, ātmā is the cause of life in a body.

But how do we know whether the reflected medium or reflected consciousness blesses the body? When the body is alive, both are present; therefore, one may argue that prāṇa etc (reflected medium) is the cause of the sentiency of the body, not ātmā (in the form of reflected consciousness). The next mantra answers it.

na prāṇena nāpānena martyo jīvati kaścana |

itareṇa tu jīvanti yasminnetāvupāśritau ||2.2.5||

No being ever lives by prāṇa nor by apāna. Rather, one lives by something else upon which these two depend. This is indeed That.

The idea that reflected consciousness gives life to the body is further clarified in this mantra with co-present and co-absent logic which conclusively establishes the cause and effect relationship. To establish a factor conclusively as the cause of an effect, the presence of the cause must always produce that effect (co-present), while its absence must always result in the absence of that effect (co-absent). Without this logic, one cannot conclusively establish the cause and effect relationship. For example, to establish that fire is the cause of smoke in the mountain, the presence of smoke must always accompany with the presence of fire, while the absence of smoke should means the absence of fire.

The body is alive when both reflected consciousness and reflected medium are present, and it dies when they both leave. Therefore, anvaya-vyatireka (co-present and co-absent) logic seems to apply also for reflected medium to be the cause of sentiency in the body. When prāṇa (represent the entire reflected medium) is present, the body is alive - co-present. When prāṇa is absent, the body is dead - co-absent.

However, prāṇa by nature is inert (made of subtle elements) and therefore, it is unable to lend sentiency to the physical body. It just functions as a medium to receive sentiency from the original consciousness and transfer it to the body in the form of reflected consciousness. Therefore śāstra says na prāṇena nāpānena martyo jīvati kaścana - no body lives by prāṇa or apāṇa (subtle body), because they act jointly for the sake of something else which is different from them; thus, the status of being the cause of life is not possible. Just like a house is brought together for the owner of the house, not the other way around. In the same way, subtle body depends on consciousness to be there, not the other way around. Therefore itareṇa tu jīvanti yasminnetāvupāśritau - a living being lives by that ātmā upon which prāṇa, apāṇan etc., depend upon. Just like to illumine a dark patch of the wall, we require both the mirror and the reflected sunlight. Only by the presence of both of them, the wall can be illumined. If we remove the mirror, the wall cannot be illumined.

Even though anvaya-vyatireka logic seems to apply in the case of mirror, it is not correct to say that mirror is the illuminator because the mirror does not have a light of its own. It is just a medium for sunlight to manifest as reflected light to illumine the wall. The illuminator is the reflected light; not the mirror. In the same way, reflected consciousness is the one who gives life, not the subtle body. This mantra helps us to knock off our wrong identification with the subtle body.

hanta te idam pravakṣyāmi guhyam brahma sanātanam |

yathā ca maraṇam prāpya ātmā bhavati gautama ||2-2-6||

Well then, oh gautama (naciketa)! I shall reveal this secret, eternal  brahman to you; and also about what happens to the ātmā after meeting death.

This is an introductory mantra, where the eternal brahman which is ātmā, the very essence of life is revealed. It is called as secret - guhyam, because it is difficult to comprehend, and sanātanam - free from time limitation. Only through the knowledge of this brahman, one is free from the life of samsāra. Therefore yamaraja reveals what would happen to the ignorant person after death (remain in the life of samsāra) if he fails to know this secret.