The Vedanta Sutras of Badarayana, Commentary by Sankara (SBE38), tr. by George Thibaut  at sacred-texts.com
49. (The view that the agnis constitute an independent vidyâ) cannot be refuted, owing to the greater force of direct enunciation and so on.
Our opponent has no right to determine, on the ground of prakarana, that the agnis are subordinate to the sacrificial action, and so to set aside our view according to which they are independent. For we know from the Pûrvâ Mîmâmsâ that direct enunciation (Sruti), indicatory mark
[paragraph continues] (liṅga), and syntactical connexion (vâkya) are of greater force than leading subject-matter (prakarana), and all those three means of proof are seen to confirm our view of the agnis being independent. In the first place we have the direct enunciation, 'These agnis are indeed knowledge-piled only.' In the second place we have the indicatory mark supplied by the passage, 'All beings ever pile for him sleeping,' &c. And in the third place we have the sentence, 'By knowledge indeed those (agnis) are piled for him who thus knows.'
In the first of these passages the emphatical expression, 'built by knowledge only,' would be contradicted if we admitted that the agnis form part of the sacrificial action.--But may this emphatical phrase not merely have the purpose of indicating that those agnis are not to be accomplished by external means?--No, we reply, for if that were intended, it would be sufficient to glorify the fact of knowledge constituting the character of the agnis by means of the word 'knowledge-piled,' and the emphatical assertion (implied in the addition of the word 'only') would be useless. For it is the nature of such agnis to be accomplished without any external means. But, although the agnis are clearly to be accomplished without external means, yet it might be supposed that, like the mental cup, they form part of the sacrificial action, and the object of the emphatical assertion implied in 'only' is to discard that suspicion.--So likewise (to pass over to liṅga) the continuity of action implied in the passage, 'For him who thus knows whether sleeping or waking all beings always pile these agnis,' is possible only on the supposition of those agnis being independent. The case is analogous to that of the imaginary agnihotra consisting of speech and breath, with reference to which the text says at first, 'He offers his breath in his speech, he offers his speech in his breath,' and then adds, 'These two endless and immortal oblations he offers always whether waking or sleeping' (Kau. Up. II, 6).--If, on the other hand, the imaginary agnis were parts of the sacrificial action it would be impossible for them to be accomplished continually, since
the accomplishment of the sacrificial action itself occupies only a short time.--Nor may we suppose the passage (which contains the liṅga) to be a mere arthavâda-passage (in which case, as the pûrvapakshin avers, the liṅga would be unable to refute prakarana). For in those cases where we meet with an unmistakeable injunctory passage--marked out as such by the use of the optative or imperative form--there indeed we may assume a glorificatory passage (met with in connexion with that injunctory passage) to be an arthavâda. In the present case, however, we observe no clear injunctory passage, and should therefore be obliged to construct one enjoining the knowledge of the various fanciful agnis, merely on the basis of the arthavâda-passage. But in that case the injunction can be framed only in accordance with the arthavâda, and as the arthavâda speaks of the continual building of the agnis, the latter item would have to appear in the injunction also. But, if so, it follows (as shown above) that the mental construction of those agnis constitutes an independent vidyâ (and does not form part of the actual agnikayana).--The same argumentation applies to the second liṅga-passage quoted above, 'Whatever those beings conceive in their minds,' &c.--And the sentence finally shows, by means of the clause, 'For him who thus knows,' that those agnis are connected with a special class of men (viz. those who thus know), and are therefore not to be connected with the sacrificial action.--For all these reasons the view of those agnis constituting an independent vidyâ is preferable.