The Vedanta Sutras of Badarayana, Commentary by Sankara (SBE38), tr. by George Thibaut  at sacred-texts.com
18. As (scripture where speaking of the rinsing of the mouth with water) makes a reference to an act (established by Smriti), (that act is not enjoined by Sruti, but rather) the new (act of meditation on the water viewed as the dress of prâna).
The Khandogas as well as the Vâgasaneyins record, in the colloquy of the prânas, that the food of Breath comprizes everything even unto dogs and birds, and that water is its dress. To this the Khandogas add, 'Therefore when going to eat food they surround it before and after with water' (Kh. Up. V, 2, 2). And the Vâgasaneyins add (Bri. Up. VI, 1, 14), 'Srotriyas who know this rinse the mouth with water when they are going to eat and rinse the mouth with water after they have eaten, thinking that thereby they make the breath dressed. Therefore a man knowing this is to rinse the mouth with water when going to eat and after having eaten; he thereby makes that breath dressed.'--These texts intimate two things, rinsing of the mouth and meditation on the breath as dressed. The doubt then arises whether the texts enjoin both these matters, or only the rinsing of the mouth, or only the meditation on breath as dressed.
The pûrvapakshin maintains that the text enjoins both, since the one as well as the other is intimated by the text, and since both matters not being settled by any other means of knowledge are worthy of being enjoined by the Veda.--Or else, he says, the rinsing of the mouth only is enjoined, since with reference to the latter only the text exhibits the particular injunctive verbal form ('he is to rinse'). In this latter case the mention made in the text of the meditation on breath as dressed has merely the purpose of glorifying the act of rinsing.
To this we make the following reply.--The rinsing of the mouth cannot possibly be enjoined by the quoted passages 'since they merely contain references to an act,' i.e. since they merely contain remarks concerning the purificatory act of rinsing the mouth which is known from and settled by Smriti.--But are not the very Sruti-passages under discussion to be looked upon as the fundamental texts on which the Smriti-injunctions regarding the rinsing of the mouth are based?--This is not possible, we reply, since the Sruti and Smriti-passages refer to different matters. All the Smriti-passages enjoin the act of rinsing the mouth only in so far as it purifies man; while the quoted Sruti texts which occur in prâna-vidyâs, if enjoining the rinsing of the mouth at all, enjoin it with reference to the knowledge of prâna. And a Sruti-passage cannot constitute the basis of a Smriti-passage referring to an altogether different matter. Nor can it be maintained that the Sruti-passage enjoins some altogether new rinsing of the mouth connected with the prâna-vidyâ, as we recognise the rinsing mentioned in Sruti as the ordinary rinsing performed by men for the sake of purification.--The preceding argumentation already precludes the alternative of two matters being enjoined, which would moreover lead to a so-called split of the sentence.--We therefore conclude that the text--with reference to the rinsing of the mouth before and after eating which is enjoined by Smriti-enjoins (by means of the passage, 'thinking that thereby they make the breath dressed') a new mental resolve with regard to the water used for rinsing purposes, viz. that that water should act as a means for clothing the prâna. The statement about the clothing of the prâna cannot (as suggested by the pûrvapakshin) be taken as a glorification of the act of rinsing the mouth; for in the first place the act of rinsing is not enjoined in the Vedic passage 1, and in the second place we apprehend that the passage itself conveys an injunction, viz. of the mental
resolve to provide clothing for the prâna. Nor must the objection be raised that in that case two purposes are admitted for the one act of rinsing the mouth, viz. the purpose of purification and the purpose of providing the prâna with clothing. For we have actually to do not with one action, but with two separate actions. For one action is the rinsing of the mouth which serves the purpose of purifying man, and another action is the mental resolve that that water should serve the purpose of clothing the prâna. Similarly the preceding passage, 'Whatever there is, even unto dogs, &c., that is thy food,' does not enjoin the promiscuous use of food of all kinds--for that would be contrary to scripture and impossible in itself--but merely enjoins the meditation on all food as food of the prâna. We therefore conclude that also the passage, 'Water is thy dress," which forms the immediate continuation of the passage last quoted does not enjoin the act of rinsing the mouth but merely the act of meditating on the rinsing-water as constituting the dress of the prâna.
Moreover the mere present-form, 'they rinse the mouth with water,' has no enjoining force.--But also in the passage, 'They think that thereby they make the breath dressed,' we have a mere present-form without injunctive power (and yet you maintain that that passage conveys an injunction)!--True; but as necessarily one of the two must be enjoined 1, we assume, on the ground of what the text says about the making of a dress, that what is enjoined is the meditation on water being the dress of prâna; for this is something 'new,' i.e. not established by other means of knowledge 2. The rinsing of the mouth with water, on the other hand, is already established by other means (i.e. Smriti), and therefore need not be enjoined again.--The argument founded
by the pûrvapakshin on the circumstance that, in the Bri. Up., the verb 'to rinse' is found in the injunctive form ('therefore a man, &c., is to rinse'), is already refuted by our showing that the act of rinsing the mouth is not a new one (and therefore requires no Vedic injunction).
For the very reason that the text does not aim at enjoining the rinsing of the mouth, the Kânvas (in their recension of the Bri. Up.) conclude the chapter with the clause, 'They think,' &c., and do not add the concluding clause of the Mâdhyandinas, 'Therefore a man,' &c. From this we have to conclude that what is enjoined in the text of the Mâdhyandinas also is 'the knowledge of that,' i.e. the knowledge of the water being the dress of the previously mentioned prâna.--Nor finally can it be maintained that in one place (i.e. the Madhyandina-sâkha) the rinsing of the mouth is enjoined, and in other places the knowledge of water as the dress of prâna; for the introductory passage, 'Water is the dress,' is the same everywhere.--We are therefore entitled to conclude that what is enjoined in all Sâkhâs is the cognition of water being the dress of the prâna.
212:1 A glorifying arthavâda-passage would be in its place only if it were preceded by some injunction; for the glorification of certain acts is meant to induce men to comply with the injunctions concerning those acts.
213:1 Because otherwise we should have only arthavâdas. But arthavâdas have a meaning only in so far as connected with an injunction.
213:2 The above argumentation avails itself of the Sûtra, putting a new construction on it.--Tarhi dvayor avidheyatvam ity âsaṅkyânu-vâdamâtrasyâऽkiñkitkaratvâd anyataravidher âvasyakatve samkalpanam eva vidheyam iti vidhântarena sûtram yogayati. Ân. Gi.