Hiraṇyagarbha & taijasa oneness

After seeing oneness between individual jīva and īśvara (causal level), now śruti presents oneness at the subtle level.

yaḥ pūrvaṃ tapaso jātamadbhyaḥ pūrvamajāyata |

guhām praviśya tiṣṭhantam yo bhūtebhirvyapaśyata |

etadvai tat ||2.1.6||

He knows (ātmā to be that hiraṇyāgarbha) who was born in the beginning from

that tapas (Īśvara), who was born before the water, (and) having entered the

heart, who resides there with the body and sense organs. This is indeed that.

The subtle universe (hiraṇyagarbha) was born out of the causal universe (Īśvara); the causal state always exists even at the time of pralaya (the unmanifest stage before creation). At the appropriate time, the causal universe evolved into the subtle universe. Thus, hiraṇyagarbha is said to be born out of Īśvara’s tapas (knowledge) at the beginning of creation - pūrvaṃ tapaso jātam.

Hiraṇyagarbha was born before the virāt principle of the gross universe - adbhyaḥ pūrvamajāyata. Adbhyaḥ means water, which represents all the five elements (paṇca bhūtāni), which in turn represent the gross universe. guhām praviśya tiṣṭhantam - (hiraṇyagarbha) having entered into the mind (of the individual), it remains as the subtle body (taijasa). The wise man understands that hiraṇyagarbha and taijasa are none other than the ātmā, naciketa asked - etad vai tat.