The Vedanta Sutras, commentary by Sankaracharya (SBE34), tr. by George Thibaut  at sacred-texts.com
26. If it be maintained that (Vaisvânara is) not (the highest Lord) on account of the term (viz. Vaisvânara, having a settled different meaning), &c., and on account of his abiding within (which is a characteristic of the gastric fire); (we say) no, on account of the perception (of the highest Lord), being taught thus (viz. in the gastric fire), and on account of the impossibility (of the heavenly world, &c. being the head, &c. of the gastric fire), and because they (the Vâgasaneyins) read of him (viz. the Vaisvânara) as man (which term cannot apply to the gastric fire).
Here the following objection is raised.--Vaisvânara cannot be the highest Lord, on account of the term, &c., and on account of the abiding within. The term, viz. the term Vaisvânara, cannot be applied to the highest Lord, because the settled use of language assigns to it a different sense. Thus, also, with regard to the term Agni (fire) in the passage (Sat. Brâ. X, 6, 1, 11), 'He is the Agni Vaisvânara.' The word '&c.' (in the Sûtra) hints at the fiction concerning the three sacred fires, the gârhapatya being represented as the heart, and so on, of the Vaisvânara Self (Kh. Up. V, 18, 2 1).--Moreover, the passage, 'Therefore the first food which a man may take is in the place of homa' (Kh. Up. V, 19,1), contains a glorification of (Vaisvânara) being the abode of the oblation to Prâna 2. For these reasons we have to understand
by Vaisvânara the gastric fire.--Moreover, Scripture speaks of the Vaisvânara as abiding within. 'He knows him abiding within man;' which again applies to the gastric fire only.--With reference to the averment that on account of the specifications contained in the passage, 'His head is Sutegas,' &c., Vaisvânara is to be explained as the highest Self, we (the pûrvapakshin) ask: How do you reach the decision that those specifications, although agreeing with both interpretations, must be assumed to refer to the highest Lord only, and not to the gastric fire?--Or else we may assume that the passage speaks of the elemental fire which abides within and without; for that that fire is also connected with the heavenly world, and so on, we understand from the mantra, 'He who with his light has extended himself over earth and heaven, the two halves of the world, and the atmosphere' (Rig-veda Samh. X, 88, 3).--Or else the attribute of having the heavenly world, and so on, for its members may, on account of its power, be attributed to that divinity which has the elemental fire for its body.--Therefore Vaisvânara is not the highest Lord.
To all this we reply as follows.--Your assertions are unfounded, 'because there is taught the perception in this manner.' The reasons (adduced in the former part of the Sûtra), viz. the term, and so on, are not sufficient to make us abandon the interpretation according to which Vaisvânara is the highest Lord.--Why?--On account of perception being taught in this manner, i.e. without the gastric fire being set aside. For the passages quoted teach the perception of the highest Lord in the gastric fire, analogously to such passages as 'Let a man meditate on the mind as Brahman' (Kh. Up. III, 18, 1).--Or else they teach that the object of perception is the highest Lord, in so far as he has the gastric fire called Vaisvânara for his limiting condition; analogously to such passages as 'He who consists of mind, whose body is breath, whose form is light' (Kh. Up. III, 14, 2 1). If it were the aim of the passages about the "Vaisvânara
to make statements not concerning the highest Lord, but merely concerning the gastric fire, there would be no possibility of specifications such as contained in the passage 'His head is Sutegas,' &c. That also on the assumption of Vaisvânara being either the divinity of fire or the elemental fire no room is to be found for the said specifications, we shall show under the following Sûtra.--Moreover, if the mere gastric fire were meant, there would be room only for a declaration that it abides within man, not that it is man. But, as a matter of fact, the Vâgasaneyins speak of him--in their sacred text--as man, 'This Agni Vaisvânara is man; he who knows this Agni Vaisvânara as man-like, as abiding within man,' &c. (Sat. Brâ. X, 6, I, II). The highest Lord, on the other hand, who is the Self of everything, may be spoken of as well as man, as abiding within man.--Those who, in the latter part of the Sûtra, read 'man-like' (purushavidham) instead of 'man' (purusham), wish to express the following meaning: If Vaisvânara were assumed to be the gastric fire only, he might be spoken of as abiding within man indeed, but not as man-like. But the Vâgasaneyins do speak of him as man-like,' He who knows him as man-like, as abiding within man.'--The meaning of the term man-like is to be concluded from the context, whence it will be seen that, with reference to nature, it means that the highest Lord has the heaven for his head, &c., and is based on the earth; and with reference to man, that he forms the head, &c., and is based on the chin (of the devout worshipper 1).
146:1 Na ka gârhapatyâdihridayâditâ brahmanah sambhavinî. Bhâmatî.
146:2 Na ka prânâhutyadhikaranatâ # nyatra gatharâgner yugyate. Bhâmatî.
147:1 According to the former explanation the gastric fire is to be looked on as the outward manifestation (pratîka) of the highest Lord; according to the latter as his limiting condition.
148:1 I.e. that he may be fancifully identified with the head and so on of the devout worshipper.