The Vedanta Sutras of Badarayana, Commentary by Sankara (SBE38), tr. by George Thibaut  at sacred-texts.com
30. The objection (raised above) is not valid, since (the connexion of the soul with the buddhi) exists as long as the soul; it being thus observed (in scripture).
We need not fear that the objection formulated above can be proved.--Why?--'On account of the existence of the connexion of the soul with the buddhi, as long as the
soul exists.' That means: as long as this Self is in the samsâra-state, as long as the samsâra-state is not brought to an end by means of perfect knowledge, so long the connexion of the soul with the buddhi does not cease. And as long as its connexion with the buddhi, its limiting adjunct, lasts, so long the individual soul remains individual soul, implicated in transmigratory existence. In reality, however, there is no individual soul but in so far as it is fictitiously hypostatized by the buddhi, its limiting adjunct. For in attempting to determine the object of the Vedânta-texts we meet with no other intelligent substance but the one omniscient Lord whose nature is eternal freedom. This appears from innumerable texts, such as the following:--'There is no other seer but he, there is no other hearer but he, there is no other perceiver but he, there is no other knower but he' (Bri. Up. III, 7, 23); 'There is nothing that sees, hears, perceives, knows but it' (Bri. Up. III, 8, 11); 'Thou art that' (Kh. Up. VI, 8, 7); 'I am Brahman' (Bri. Up. I, 4, 10).--How again is it known that the soul is connected with the buddhi as long as it exists?--We reply: because that is seen (viz. in scripture). For scripture makes the following declaration: 'He who is within the heart, consisting of knowledge, surrounded by the prânas, the person of light, he remaining the same wanders along the two worlds as if thinking, as if moving' (Bri. Up. IV, 3, 7). Here the term 'consisting of knowledge' means 'consisting of buddhi,' as we infer from another passage, viz. 'The Self consisting of knowledge, mind, life, sight, hearing' (Bri. Up. IV, 4, 5), where knowledge is enumerated among mind and, so on 1. By 'being made up of buddhi' is meant 'having for one's essence the qualities of buddhi.' Similarly a phrase like 'Devadatta is made up of womanishness,' which may be made use of in ordinary language, means that in Devadatta feminine attributes such as softness of voice and the like prevail. Moreover, the passage, 'He remaining the same wanders along the two worlds,' declares that the Self, even
when going to another world, is not separated from the buddhi, &c. For if we ask whereby it does remain the same, the answer, based on proximity 1, is 'by means of the buddhi.'--Further, such modes of expression, 'as if thinking,' 'as if moving,' lead us to the same conclusion; for they mean that the Self does not think and move on its own account, but thinks as it were and moves as it were, because the buddhi to which it is joined really moves and thinks.--Moreover, the connexion of the Self with the buddhi, its limiting adjunct, depends on wrong knowledge, and wrong knowledge cannot cease except through perfect knowledge; hence as long as there does not rise the cognition of Brahman being the universal Self, so long the connexion of the soul with the buddhi and its other limiting adjuncts does not come to an end. Thus scripture also says, 'I know that great person of sunlike lustre beyond the darkness. A man who knows him passes over death; there is no other path to go' (Sve. Up. III, 8).
But, an objection is raised, in the states of deep sleep and retractation (pralaya) no connexion of the Self with the buddhi can be acknowledged, since scripture declares that 'then he becomes united with the True, he is gone to his own' (Kh. Up. VI, 8, 1), and as then all modifications have avowedly passed away. How then can it be said that the connexion with the buddhi exists as long as the Self?--To this objection the following Sûtra replies.
46:1 And therefore has to be understood in the sense of buddhi.
47:1 I.e. on the proximity of terms clearly indicating the buddhi, viz. vigñâna-mayah prâneshu.