How can the Vedas be authorless when it has authored speech? For example in the Chandogya Upanishad:
The Devas (gods) and Asuras (demons) both heard these words, and said: 'Well, let us search for that Self by which, if one has searched it out, all worlds and all desires are obtained.'Thus saying Indra went from the Devas, Virokana from the Asuras, and both, without having communicated with each other, approached Pragâpati, holding fuel in their hands, as is the custom for pupils approaching their master. They dwelt there as pupils for thirty-two years. Then Pragâpati asked them: 'For what purpose have you both dwelt here?'
The Upanishads are full of conversations like this. So how can it be said that the Vedas are eternal when it consists of authored speech?
Again and again the Upaniṣads glorify Self-knowledge, but what is the nature of the Self, and how do we attain that knowledge? Here the Upaniṣad begins a story to answer this. Once Prajāpati, the creator, decided to teach people about the Self. He described the Self as apahata-pāpmā, free from sins, or blemishes (pāpa)—that is to say, it is pure. Vijara—it never ages, or decays. Vimṛtyu—it is free from death.
All similar conversations are either stories or allegories that are intended to teach about either brahman or details of yagna etc. These are not to be considered as real conversations.
(I don't have scriptural reference, so I'll repeat what we discussed in comments earlier based on purely logical stance)
--The hidden premise in your question --
How can Vedas be Apaurusheya IF they contain authored conversations ?
--seems to be that--
Vedas can be Apaurusheya IF THEY DO NOT contain authored conversations.
Vedas : "Prajapati said 'Search for Brahman/Atman'"
Vedas : "Brahma created worlds as it was before (Yatha Purvam Akalpayat)"
You consider the 1st statement to be 'authored speech' i.e. dependent on a person/time/place - hence, man-made, hence against the doctrine of having no creator (Apaurusheya).
You consider the 2nd statement as possibly independent, because Brahma is a post inhabited by different Jivas in each kalpa, hence the same cycle of creation repeats, hence not against the doctrine of Apaurusheya.
My stance is to establish an equivalence between these two.
The rishis/devas/asuras/indra/prajapati - could be posts too. The exact same conversations could happen in every kalpa, just as the exact same creation happens in every kalpa.
Or - Brahma could be as mortal as the authors of those conversations. There is no need to grant special status to the Jiva Brahma, if you are not granting it to the speakers.
Any statement in Vedas refers to some being or thing or action in the universe.
If Vedas refer to a tree (a Jiva), or Brahma (a Jiva), or Prajapati (a Jiva), or a tree listening to Vedas (an action by a Jiva), or Brahma creating the world (an action by a Jiva), or Prajapati giving some advice (an action by a Jiva) - I do not see any difference.
How can Vedas be Apaurusheya IF they contain anything that exists in this universe ?
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