The Vedanta Sutras, commentary by Sankaracharya (SBE34), tr. by George Thibaut  at sacred-texts.com
15. And on account of the passage referring to that which is distinguished by pleasure (i.e. Brahman).
There is, moreover, really no room for dispute whether Brahman be meant in the passage under discussion or not, because the fact of Brahman being meant is established 'by the reference to that which is distinguished by pleasure.' For the same Brahman which is spoken of as characterised by pleasure in the beginning of the chapter 2, viz. in the clauses, 'Breath is Brahman, Ka is Brahman, Kha is Brahman,' that same Brahman we must suppose to be referred to in the present passage also, it being proper to adhere to the subject-matter under discussion; the clause, 'The teacher will tell you the way 3,' merely announcing that the way will be proclaimed [by the teacher; not that a new subject will be started].--How then, it may be asked, is it known that Brahman, as distinguished by pleasure, is spoken of in the beginning of the passage?--We reply: On hearing the speech of the fires, viz. 'Breath is Brahman, Ka is Brahman, Kha is Brahman,' Upakosala says, 'I understand that breath is Brahman, but I do not understand that Ka or Kha is Brahman.' Thereupon the fires reply, 'What is Ka is Kha, what is Kha is Ka.' Now the word Kha denotes in ordinary language the elemental ether. If therefore the word Ka which means pleasure were not applied to qualify the sense of 'Kha,' we should conclude
that the name Brahman is here symbolically 1 given to the mere elemental ether as it is (in other places) given to mere names and the like. Thus also with regard to the word Ka, which, in ordinary language, denotes the imperfect pleasure springing from the contact of the sense-organs with their objects. If the word Kha were not applied to qualify the sense of Ka we should conclude that ordinary pleasure is here called Brahman. But as the two words Ka and Kha (occur together and therefore) qualify each other, they intimate Brahman whose Self is pleasure. If 2 in the passage referred to (viz. 'Breath is Brahman, Ka is Brahman, Kha is Brahman') the second Brahman (i.e. the word Brahman in the clause 'Ka is Brahman ') were not added, and if the sentence would run 'Ka, Kha is Brahman,' the word Ka would be employed as a mere qualifying word, and thus pleasure as being a mere quality would not be represented as a subject of meditation. To prevent this, both words--Ka as well as Kha--are joined with the word Brahman ('Ka (is) Brahman, Kha (is) Brahman'). For the passage wishes to intimate that pleasure also, although a quality, should be meditated upon as something in which qualities inhere. It thus appears that at the beginning of the chapter Brahman, as characterised by pleasure, is spoken of. After that the Gârhapatya and the other sacred fires proclaim in turns their own glory, and finally conclude with the words, 'This is our knowledge, O friend, and the knowledge of the Self;' wherein they point back to the Brahman spoken of before. The words, 'The teacher will tell you the way' (which form the last clause of the concluding passage), merely promise an explanation of the way, and thus preclude the idea of another topic being started. The teacher thereupon saying, 'As water does not cling to a lotus leaf, so no evil deed clings to one who knows it' (which words intervene between the concluding
speech of the fires and the information given by the teacher about the person within the eye) declares that no evil attacks him who knows the person within the eye, and thereby shows the latter to be Brahman. It thus appears that the teacher's intention is to speak about that Brahman which had formed the topic of the instruction of the fires; to represent it at first as located in the eye and possessing the qualities of Samyadvâma and the like, and to point out afterwards that he who thus knows passes on to light and so on. He therefore begins by saying, 'That person that is seen in the eye that is the Self.'
126:1 The tîkâs say that the contents of this last sentence are hinted at by the word 'and' in the Sûtra.
126:2 I.e. at the beginning of the instruction which the sacred fires give to Upakosala, Kh. Up. IV, 10 ff.
126:3 Which words conclude the instruction given by the fires, and introduce the instruction given by the teacher, of which the passage 'the person that is seen in the eye,' &c. forms a part.
127:1 Âsrayântarapratyayasyâsrayântare kshepah pratîkah, yathâ brahmasabdah paramâtmavishayo nâmâdishu kshipyate. Bhâ.
127:2 The following sentences give the reason why, although there is only one Brahman, the word Brahman is repeated.