The Vedanta Sutras of Badarayana, Commentary by Sankara (SBE38), tr. by George Thibaut  at sacred-texts.com
52. And from the subsequent (Brâhmana) it follows that being of that kind (i.e. injunction of a mere vidyâ) (is the aim) of the text. The connexion (of the fanciful agnis with the real one) is due to the plurality (of details of the real agni which are imaginatively connected with the vidyâ).
With regard to a subsequent Brâhmana also, viz. the one beginning, 'That piled agni is this world indeed,' we apprehend that what is the purpose of the text is 'being of that kind,' i.e. injunction of a mere vidyâ, not injunction of the member of a mere action. For we meet there with the following sloka,' By knowledge they ascend there where all wishes are attained. Those skilled in works do not go there, nor those who destitute of knowledge do penance.' This verse blames mere works and praises knowledge. A former Brâhmana also, viz. the one beginning, 'What that orb leads' (Sat. Brâ. X, 5, 2, 23), concludes with a statement of the fruit of knowledge ('Immortal becomes he whose Self is death'), and thereby indicates that works are not the chief thing.--The text connects the vidyâ (of the agnis built of
mind) with the real agni built of bricks, not because those agnis are members of the act of building the real agni, but because many of the elements of the real agni are imaginatively combined with the vidyâ.
All this establishes the conclusion that the fire-altars built of mind and so on constitute a mere vidyâ.