The Vedanta Sutras of Badarayana, Commentary by Sankara (SBE38), tr. by George Thibaut  at sacred-texts.com
44. On account of the majority of indicatory marks (the fire-altars built of mind, &c. do not form elements of any act); for this (i.e. the indicatory
mark) is stronger (than the general subject-matter); this also (has been explained in the Pû. Mî. Sûtras).
In the Agnirahasya of the Vâgasaneyins, in the Brâhmana beginning 'for in the beginning indeed this was not existent,' we read with reference to mind (manas),' It saw thirty-six thousand shining fire-altars, belonging to itself, made of mind, built of mind.' And, further on, the text makes similar statements about other fanciful fire-altars built of speech, built of breath, built of sight, built of hearing, built of work, built of fire.--A doubt here arises whether these fire-altars built of mind and so on are connected with the act (i.e. the construction of the fire-altar made of bricks), arid supplementary to it, or whether they are independent, constituting a mere vidyâ.
Against the prima facie view that those agnis are connected with the sacrificial act under whose heading the text records them, the Sûtra maintains their independence, 'on account of the majority of indicatory marks.' For we meet in that Brâhmana with a number of indicatory marks confirming that those agnis constitute a mere vidyâ; cp, e.g. the following passages: 'Whatever these beings conceive in their minds, that is a means for those fire-altars,' and 'All beings always pile up those fire-altars for him who thus knows, even when he sleeps,' and so on 1.--And that indicatory marks (liṅga) are of greater force than the leading subject-matter (prakarana) has been explained in the Pûrva Mîmâmsâ (III, 3, 14).
260:1 For something which forms part of an act cannot be brought about by something so indefinite as 'whatever these beings conceive in their minds,' nor can it be accomplished indifferently at any time by any beings.