The Vedanta Sutras, commentary by Sankaracharya (SBE34), tr. by George Thibaut  at sacred-texts.com
20. And the embodied soul (also cannot be understood by the internal ruler), for both also (i. e. both
recensions of the Brihad Âranyaka) speak of it as different (from the internal ruler).
The word 'not' (in the Sûtra) has to be supplied from the preceding Sûtra. Although the attributes of seeing, &c., belong to the individual soul, still as the soul is limited by its adjuncts, as the ether is by a jar, it is not capable of dwelling completely within the earth and the other beings mentioned, and to rule them. Moreover, the followers of both sâkhâs, i.e. the Kânvas as well as the Mâdhyandinas, speak in their texts of the individual soul as different from the internal ruler, viz. as constituting, like the earth, and so on, his abode and the object of his rule. The Kânvas read (Bri. Up. III, 7, 22), 'He who dwells in knowledge;' the Mâdhyandinas, 'He who dwells in the Self.' If the latter reading is adopted, the word 'Self' denotes the individual soul; if the former, the individual soul is denoted by the word 'knowledge;' for the individual soul consists of knowledge. It is therefore a settled matter that some being different from the individual soul, viz. the lord, is denoted by the term 'internal ruler.'--But how, it may be asked, is it possible that there should be within one body two seers, viz. the lord who rules internally and the individual soul different from him?--Why--we ask in return--should that be impossible?--Because, the opponent replies, it is contrary to scriptural passages, such as, 'There is no other seer but he,' &c., which deny that there is any seeing, hearing, perceiving, knowing Self, but the internal ruler under discussion.--May, we rejoin, that passage not have the purpose of denying the existence of another ruler?--No, the opponent replies, for there is no occasion for another ruler (and therefore no occasion for denying his existence), and the text does not contain any specification, (but merely denies the existence of any other seer in general.)
We therefore advance the following final refutation of the opponent's objection.--The declaration of the difference of the embodied Self and the internal ruler has its reason in the limiting adjunct, consisting of the organs of action, presented by Nescience, and is not absolutely true. For the
[paragraph continues] Self within is one only; two internal Selfs are not possible. But owing to its limiting adjunct the one Self is practically treated as if it were two; just as we make a distinction between the ether of the jar and the universal ether. Hence there is room for those scriptural passages which set forth the distinction of knower and object of knowledge, for perception and the other means of proof, for the intuitive knowledge of the apparent world, and for that part of Scripture which contains injunctions and prohibitions. In accordance with this, the scriptural passage, 'Where there is duality, as it were, there one sees another,' declares that the whole practical world exists only in the sphere of Nescience; while the subsequent passage, 'But when the Self only is all this, how should he see another?' declares that the practical world vanishes in the sphere of true knowledge.