The Vedanta Sutras of Badarayana, Commentary by Sankara (SBE38), tr. by George Thibaut  at sacred-texts.com
5. If an objection be raised on the ground of (water) not being mentioned in the first fire, we refute it by remarking that just it (viz. water) (is meant), on the ground of fitness.
Well, the pûrvapakshin resumes, but how can it be ascertained that 'in the fifth oblation water is called man,' considering that water is not mentioned by scripture with reference to the first fire (altar)? For the text enumerates five fires--the first of which is the heavenly world--as the abodes of the five oblations. With reference to the first of those fires--introduced by the words 'The fire is that world, O Gautama,' it is stated that sraddhâ (faith) is the material constituting the oblation ('on that altar the devas offer sraddhâ'); while nothing is said about water being the offered material. If, with reference to the four following fires, viz. Parganya, &c., water is assumed to constitute the offering, we have no objection because in the substances stated there as forming the oblations, viz. Soma, and so on, water may preponderate. But to set aside, in the case of
the first fire, sraddhâ (i.e. faith) which is directly mentioned in the text, and to substitute in its place the assumption of water, about which the text says nothing, is an arbitrary proceeding. In reality sraddhâ be explained, in conformity with its ordinary meaning, as a kind of mental state, viz. faith. Hence it is objectionable to maintain that water, in the fifth oblation, becomes man.
To this view of the pûrvapakshin we demur, because, in the case of the first fire, the word sraddhâ is to be taken in the sense of 'water.'--On what ground?--On the ground of fitness. For on that explanation only beginning, middle, and end of the passage harmonise so that the syntactical unity of the whole remains undisturbed. On the other explanation (i.e. sraddhâ being taken in the sense of 'faith'), if the question were asked how water, in the fifth oblation, can be called man, and if, in way of reply, the text could point only to faith, i.e. something which is not water, as constituting the material of the oblation; then question and answer would not agree, and so the unity of the whole passage would be destroyed. The text, moreover, by concluding 'For this reason is water in the fifth oblation called man,' indicates the same interpretation 1.--Further, the text points out, as effects of sraddhâ, substances in which water in its gross form preponderates, viz. Soma, rain, &c. And this again furnishes a reason for interpreting sraddhâ as water, because the effect generally is cognate in nature to the cause. Nor again can the mental conception called faith be taken out from the mind or soul, whose attribute it is, and be employed as an offering, as the heart can be cut out of the sacrificial animal. For this reason also the word sraddhâ be taken to mean 'water.' Water can, moreover, be fitly called by that name, on the ground of Vedic usage, cp. 'sraddhâ is indeed is water' (Taitt. Samh. I, 6, 8, 1). Moreover, water when forming the seed of the body enters into the state of thinness, subtilty, and herein again resembles faith, so that its being called sraddhâ
is analogous to the case of a man who is as valiant as a lion being himself called a lion.--Again, the word sraddhâ may fitly be applied to water, because water is intimately connected with religious works (sacrifices, &c.) which depend on faith; just as the word 'platform' is applied to men (standing on the platform). And finally the waters may fitly be called sraddhâ, on account of their being the cause of faith, according to the scriptural passage, 'Water indeed produces faith in him for holy works 1.'
107:1 Upasamhârâlokanâyâm api sraddhâsabdatvam apâm evety âha tv iti. Ân. Gi.
108:1 Âpo heti, asmai pumseऽdhikârine samnamante ganayanti darsanamâtrena snânâdipunyakarmasiddhyartham sraddhâm ity arthah. Ân. Gi.