Is Krishna mentioned in the Chandogya Upanishad?
Chandogya Upanishad declaration on Ghora Angirasa and Devaki Putra Krishna:
taddhaitadghor āṅgirasaḥ kṛṣṇāya devakīputrāyoktvovācāpipāsa eva sa babhūva so'ntavelāyāmetattrayaṃ pratipadyetākṣitamasyacyutamasi prāṇasam̐śitamasīti tatraite dve ṛcau bhavataḥ ॥
The meaning of this mantra as given by shrI shankara and shrI madhva differs from that of shrI rAmAnuja. shrI shankara says:
"Ghora Angirasa, after having communicated this (view of the sacrifice) to Krishna, the son of Devaki -- and he never thirsted again (after other knowledge)--said: 'Let a man, when his end approaches, take refuge with this Triad: "Thou art the imperishable," "Thou art the unchangeable," "Thou art the edge of Prâna."' (source: net translation of shankara bhAshya)
Note that here, "Krishna, the Son of Devaki" is considered a student of Ghora Angirasa according to this interpretation. shrI madhva gives the same interpretation and he adds, "Vasudeva’s son, Krishna was the resplendent Lord himself, while there was some one else who was the son of one named Devaki, with the same name who was an ascetic. " (Source: Sonde's publication of mAdhva bhAshya).
While shrI shankara does not identify who "devaki putra krishNa" is, it is possible he regarded him as saguNa brahman krishNa himself. Because krishNa being a student does not reduce his brahmatva. Or, maybe shankara thought he was someone else. The fact that he didn't comment shows the acharya's lack of interest in the issue anyway. madhva sampradaya strongly hold the view that it was some rishi of the same name as bhagavAn.
Now, shrI ranga rAmAnuja muni, who belongs to the vishishtadvaita school and who has faithfully adhered to the view of shrI rAmAnuja, disagrees with both the interpretation of the upanishad by the other two gurus and also the idea that "devaki putra krishNa" is someone other than bhagavAn. There is really no need to take it as any other person considering that the theme of the upanishad is "upAsaNa" and the reference to "devaki putra" could easily be interpreted as an indication of the auspicious guNas like sousIlya, saulabhya, etc of the Lord which are important for upAsaNa.
srI ranga rAmAnuja muni derives the meaning another way as follows:
"The rishi Ghora Angirasa practiced this Purusha Yajna with the dedication as "This is sub-servient to Krishna, the Son of Devaki". That Ghora Angirasa had not thirst, as he came upon BrahmavidyA through this. At the last moment of his life, he said to Brahman, "you are eternal, you are full of auspicious qualities, you are the subtle truth enlivening this universe".
The acharyan explains "KrishnAya" as "KrishnaseshabhUta" - for the sake of Krishna (the essential nature of the jIvAtmAn is seshatva or servitude to devaki putra krishNa).
One must note that even in mantrAs, the "Aya" shabda denotes "for the sake of-", or service performed for the deity.
ityuktvA means anusandhAna, which is upAsaNa. Constant meditation on devaki putra krishNa and oneself as being his dAsabhUta by nature is thus recommended. Jiva Goswami also alludes to this interpretation in his commentary on Brahma Samhita.
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