The Vedanta Sutras, commentary by Sankaracharya (SBE34), tr. by George Thibaut  at sacred-texts.com
42. And (on account of the designation) (of the highest Self) as different (from the individual soul) in the states of deep sleep and departing.
In the sixth prapâthaka of the Brihadâranyaka there is given, in reply to the question, 'Who is that Self?' a lengthy exposition of the nature of the Self, 'He who is within the heart, among the prânas, the person of light, consisting of knowledge' (Bri. Up. IV, 3, 7). Here the doubt arises, whether the passage merely aims at making an additional statement about the nature of the transmigrating soul (known already from other sources), or at establishing the nature of the non-transmigrating Self.
The pûrvapakshin maintains that the passage is concerned with the nature of the transmigrating soul, on account of the introductory and concluding statements. For the introductory statement, 'He among the prânas who consists of knowledge,' contains marks indicatory of the embodied soul, and so likewise the concluding passage, 'And that great unborn Self is he who consists of cognition,' &c. (IV, 4, 22). We must therefore adhere to the same subject-matter in the intermediate passages also, and look on them as setting forth the same embodied Self, represented in its different states, viz. the waking state, and so on.
In reply to this, we maintain that the passage aims only at giving information about the highest Lord, not at making additional statements about the embodied soul.--Why?--On account of the highest Lord being designated as different from the embodied soul, in the states of deep sleep and of departing from the body. His difference from the embodied soul in the state of deep sleep is declared in the following passage, 'This person embraced by the intelligent (prâgña) Self knows nothing that is without, nothing that is within.' Here the term, 'the person,' must mean the embodied soul; for of him it is possible to deny that he knows, because he, as being the knower, may know what is within and without. The 'intelligent Self,' on the other hand, is the highest Lord, because he is never dissociated from intelligence, i.e.--in his case--all-embracing knowledge.--Similarly, the passage treating of departure, i.e. death ('this bodily Self mounted by the intelligent Self moves along groaning'), refers to the highest Lord as different from the individual Self. There also we have to understand by the 'embodied one' the individual
soul which is the Lord of the body, while the 'intelligent one' is again the Lord. We thus understand that 'on account of his being designated as something different, in the states of deep sleep and departure,' the highest Lord forms the subject of the passage.--With reference to the pûrvapakshin's assertion that the entire chapter refers to the embodied Self, because indicatory marks of the latter are found in its beginning, middle, and end, we remark that in the first place the introductory passage ('He among the prânas who consists of cognition') does not aim at setting forth the character of the transmigrating Self, but rather, while merely referring to the nature of the transmigrating Self as something already known, aims at declaring its identity with the highest Brahman; for it is manifest that the immediately subsequent passage, 'as if thinking, as if moving 1, 'aims at discarding the attributes of the transmigrating Self. The concluding passage again is analogous to the initial one; for the words, 'And that great unborn Self is he who,' &c., mean: We have shown that that same cognitional Self, which is observed among the prânas, is the great unborn Self, i.e. the highest Lord--He, again, who imagines that the passages intervening (between the two quoted) aim at setting forth the nature of the transmigrating Self by representing it in the waking state, and so on, is like a man who setting out towards the east, wants to set out at the same time towards the west. For in representing the states of waking, and so on, the passage does not aim at describing the soul as subject to different states or transmigration, but rather as free from all particular conditions and transmigration. This is evident from the circumstance that on Ganaka's question, which is repeated in every section, 'Speak on for the sake of emancipation,' Yâgñavalkya replies each time, 'By all that he is not affected, for that person is not attached to anything' (Bri. Up. IV, 3, 14-16). And later on he says (IV, 3, 22), 'He is not followed by
good, not followed by evil, for he has then overcome all the sorrows of the heart.' We have, therefore, to conclude that the chapter exclusively aims at setting forth the nature of the non-transmigrating Self.
43. And on account of such words as Lord, &c.
That the chapter aims at setting forth the nature of the non-transmigrating Self, we have to conclude from that circumstance also that there occur in it terms such as Lord and so on, intimating the nature of the non-transmigrating Self, and others excluding the nature of the transmigrating Self. To the first class belongs, for instance, 'He is the lord of all, the king of all things, the protector of all things.' To the latter class belongs the passage, 'He does not become greater by good works, nor smaller by evil works.'--From all which we conclude that the chapter refers to the non-transmigrating highest Lord.
235:1 The stress lies here on the 'as if.' which intimate that the Self does not really think or move.