The Vedanta Sutras, commentary by Sankaracharya (SBE34), tr. by George Thibaut  at sacred-texts.com
12. And on account of the distinctive qualities (mentioned).
Moreover, the distinctive qualities mentioned in the text agree only with the individual Self and the highest Self. For in a subsequent passage (I, 3, 3), 'Know the Self to be the charioteer, the body to be the chariot,' which contains the simile of the chariot, the individual soul is represented as a charioteer driving on through transmigratory existence and final release, while the passage (9), 'He reaches the end of his journey, and that is the highest place of Vishnu,' represents the highest Self as the goal of the driver's course. And in a preceding passage also, (I, 2, 12, 'The wise, who by means of meditation on his Self, recognises the Ancient who is difficult to be seen, who has entered into the dark, who is hidden in the cave, who dwells in the abyss, as God, he indeed leaves joy and sorrow far behind,') the same two beings are distinguished as thinker and as object of thought. The highest Self is, moreover, the general topic. And further, the clause, 'Those who know Brahman call them,' &c., which brings forward a special class of speakers, is in its place only if the highest Self is accepted (as one of the two beings spoken of). It is therefore evident that the passage under discussion refers to the individual soul and the highest Self.
The same reasoning applies to the passage (Mu. Up. III, 1, 1), 'Two birds, inseparable friends,' &c. There also the Self is the general topic, and hence no two ordinary birds can be meant; we therefore conclude from the characteristic mark of eating, mentioned in the passage, 'One of them eats the sweet fruit,' that the individual soul is meant, and from
the characteristic marks of abstinence from eating and of intelligence, implied in the words, 'The other looks on without eating,' that the highest Self is meant. In a subsequent mantra again the two are distinguished as the seer and the object of sight. 'Merged into the same tree (as it were into water) man grieves at his own impotence (anîsâ), bewildered; but when he sees the other Lord (îsa) contented and knows his glory, then his grief passes away.'
Another (commentator) gives a different interpretation of the mantra, 'Two birds inseparable,' &c. To that mantra, he says, the final decision of the present head of discussion does not apply, because it is differently interpreted in the Paingi-rahasya Brâhmana. According to the latter the being which eats the sweet fruit is the sattva; the other being which looks on without eating, the individual soul (gña); so that the two are the sattva and the individual soul (kshetragña). The objection that the word sattva might denote the individual soul, and the word kshetragña, the highest Self, is to be met by the remark that, in the first place, the words sattva and kshetragña have the settled meaning of internal organ and individual soul, and are in the second place, expressly so interpreted there, (viz. in the Paingi-rahasya,) 'The sattva is that by means of which man sees dreams; the embodied one, the seer, is the kshetragña; the two are therefore the internal organ and the individual soul.' Nor does the mantra under discussion fall under the pûrvapaksha propounded above. For it does not aim at setting forth the embodied individual soul, in so far as it is characterised by the attributes connected with the transmigratory state, such as acting and enjoying; but in so far rather as it transcends all attributes connected with the samsâra and is of the nature of Brahman, i.e. is pure intelligence; as is evident from the clause, 'The other looks on without eating.' That agrees, moreover, with sruti and smriti passages, such as, 'That art thou,' and 'Know me also to be the individual soul' (Bha. Gîtâ XIII, 2). Only on such an explanation of the passage as the preceding one there is room for the declaration made in the concluding passage of the section, 'These two are the sattva and the kshetragña; to him indeed
who knows this no impurity attaches 1.--But how can, on the above interpretation, the non-intelligent sattva (i.e. the internal organ) be spoken of as an enjoyer, as is actually done in the clause, 'One of them eats the sweet fruit?'--The whole passage, we reply, does not aim at setting forth the fact that the sattva is an enjoyer, but rather the fact that the intelligent individual soul is not an enjoyer, but is of the nature of Brahman. To that end 2 the passage under discussion metaphorically ascribes the attribute of being an enjoyer to the internal organ, in so far as it is modified by pleasure, pain, and the like. For all acting and enjoying is at the bottom based on the non-discrimination (by the soul) of the respective nature of internal organ and soul; while in reality neither the internal organ nor the soul either act or enjoy; not the former, because it is non-intelligent; not the latter, because it is not capable of any modification. And the internal organ can be considered as acting and enjoying, all the less as it is a mere presentment of Nescience. In agreement with what we have here maintained, Scripture ('For where there is as it were duality there one sees the other,' &c.; Bri. Up. IV, 5, 15) declares that the practical assumption of agents, and so on--comparable to the assumption of the existence of elephants, and the like, seen in a dream--holds good in the sphere of Nescience only; while the passage, 'But when the Self only is all this, how should he see another?' declares that all that practically postulated existence vanishes for him who has arrived at discriminative knowledge.
123:1 Freedom from impurity can result only from the knowledge that the individual soul is in reality Brahman. The commentators explain ragas by avidyâ.
123:2 Tadartham iti, gîvasya brahmasiddhyartham iti yâvat, kaitanyakhâyâpannâ dhîh sukhâdinâ parinamata iti, tatra purusho#pi bhaktritvam ivânubhavati na tattvata iti vaktum adhyâropayati. Ânanda Giri.