The Vedanta Sutras, commentary by Sankaracharya (SBE34), tr. by George Thibaut  at sacred-texts.com
9. On account of (the individual Soul) going to the Self (the Self cannot be the pradhâna).
With reference to the cause denoted by the word 'Sat,' Scripture says, 'When a man sleeps here, then, my dear, he becomes united with the Sat, he is gone to his own (Self). Therefore they say of him, "he sleeps" (svapiti), because he is gone to his own (svam apîta).' (Kh. Up. VI, 8, 1.) This passage explains the well-known verb 'to sleep,' with reference to the soul. The word, 'his own,' denotes the Self which had before been denoted by the word Sat; to the Self he (the individual soul) goes, i.e. into it it is resolved, according to the acknowledged sense of api-i, which means 'to be resolved into.' The individual soul (gîva) is called awake as long as being connected with the various external objects by means of the modifications of the mind--which thus constitute limiting adjuncts of the soul--it apprehends those external objects, and identifies itself with the gross body, which is one of those external objects 1. When, modified by the impressions which the external objects have left, it sees dreams, it is denoted by the term 'mind 2.' When, on the cessation of the two limiting adjuncts (i.e. the subtle and the gross bodies), and the consequent absence of the modifications due to the adjuncts, it is, in the state of deep sleep, merged in the Self as it were, then it is said to be asleep (resolved into the Self). A similar etymology of the word 'hridaya' is given by sruti, 'That Self abides in the heart. And this is the etymological explanation: he is in the heart (hridi ayam).' (Kh. Up. VIII, 3, 3.) The words asanâya and udanyâ are similarly etymologised: 'water is carrying away what has been eaten by him;' 'fire carries away what has been drunk by him' (Kh. Up. VI, 8, 3; 5). Thus the passage quoted above explains the resolution (of the soul) into the Self, denoted by the term 'Sat,' by means of the etymology of the word 'sleep.' But the intelligent
[paragraph continues] Self can clearly not resolve itself into the non-intelligent pradhâna. If, again, it were said that the pradhâna is denoted by the word 'own,' because belonging to the Self (as being the Self's own), there would remain the same absurd statement as to an intelligent entity being resolved into a non-intelligent one. Moreover another scriptural passage (viz. 'embraced by the intelligent--prâgña--Self he knows nothing that is without, nothing that is within,' Bri. Up. IV, 3, 21) declares that the soul in the condition of dreamless sleep is resolved into an intelligent entity. Hence that into which all intelligent souls are resolved is an intelligent cause of the world, denoted by the word 'Sat,' and not the pradhâna.--A further reason for the pradhâna not being the cause is subjoined.
59:1 So according to the commentators, not to accept whose guidance in the translation of scholastic definitions is rather hazardous. A simpler translation of the clause might however be given.
59:2 With reference to Kh. Up. VI, 8, 2.