The Vedanta Sutras of Badarayana, Commentary by Sankara (SBE38), tr. by George Thibaut [1896] at sacred-texts.com
14. But there is guidance (of the prânas) by fire, &c., on account of that being declared by scripture.
Here there arises a discussion whether the prânas of which we have been treating are able to produce their effects by their own power or only in so far as guided by divinities.--The pûrvapakshin maintains that the prânas being endowed with the capacity of producing their effects act from their own power. If we, moreover, admitted that the prânas act only in so far as guided by divinities, it would follow that those guiding divinities are the enjoyers (of the fruits of the actions), and the individual soul would thus cease to be an enjoyer. Hence the prânas act from their own power.--To this we reply as follows. 'But there takes place guidance by fire,' &c.--The word 'but' excludes the pûrvapaksha. The different classes of organs, speech, &c., the Sûtra says, enter on their peculiar activities, guided by the divinities animating fire, and so on. The words, 'on account of that being declared by scripture,' state the reason. For different passages declare this, cp. Ait. Âr, II, 4, 2, 4, 'Agni having become speech entered the mouth.' This statement about Agni (fire) becoming speech and entering the mouth is made on the assumption of Agni acting as a ruler with his divine Self (not as a mere element). For if we abstract from the connexion with the divinity we do not see that there is any special connexion of fire either with speech or the mouth. The subsequent passages, 'Vâyu having become breath entered into the nostrils,' &c, are to be explained in the same way.
[paragraph continues] --This conclusion is confirmed by other passages also, such as 'Speech is indeed the fourth foot of Brahman; that foot shines with Agni as its light and warms' (Kh. Up. IV, 18, 3), which passage declares that speech is made of the light of Agni. Other passages intimate the same thing by declaring that speech, &c., pass over into Agni, &c., cp. Bri. Up. I, 3, 12, 'He carried speech across first; when speech had become freed from death it became Agni.' Everywhere the enumeration of speech and so on on the one side and Agni and so on on the other side--wherein is implied a distinction of the personal and the divine element--proceeds on the ground of the same relation (viz. of that which is guided and that which guides). Smriti-passages also declare at length that speech, &c., are guided by Agni and the other divinities, cp. for instance,' Brâhmanas knowing the truth call speech the personal element, that which is spoken the natural element and fire (Agni) the divine element.'--The assertion that the prânas being endowed with the capability of producing their effects act from their own power is unfounded, as we see that some things which possess the capability of motion, e.g. cars, actually move only if dragged by bulls and the like. Hence, as both alternatives are possible 1, we decide on the ground of scripture that the prânas act under the guidance of the divinities.--The next Sûtra refutes the assertion that from the fact of the divinities guiding the prânas it would follow that they--and not the embodied soul--are the enjoyers.
92:1 Viz. that something should act by itself, and that it should act under guidance only.