The Vedanta Sutras, commentary by Sankaracharya (SBE34), tr. by George Thibaut  at sacred-texts.com
18. If it be said that the other one (i.e. the individual soul) (is meant) on account of a reference to it (made in a complementary passage), (we say) no, on account of the impossibility.
If the small (ether) is to be explained as the highest Lord on account of a complementary passage, then, the pûrvapakshin resumes, we point out that another complementary passage contains a reference to the other one, i. e. to the individual soul: 'Now that serene being (literally: serenity, complete satisfaction), which after having risen out from this earthly body and having reached the highest light, appears in its true form, that is, the Self; thus he spoke' (Kh. Up. VIII, 3, 4). For there the word 'serenity,' which is known to denote, in another scriptural passage, the state of deep sleep, can convey the idea of the individual soul only when it is in
that state, not of anything else. The 'rising from the body' also can be predicated of the individual soul only whose abode the body is; just as air, &c., whose abode is the ether, are said to arise from the ether. And just as the word 'ether,' although in ordinary language not denoting the highest Lord, yet is admitted to denote him in such passages as, 'The ether is the revealer of forms and names,' because it there occurs in conjunction with qualities of the highest Lord, so it may likewise denote the individual soul Hence the term 'the small ether' denotes in the passage under discussion the individual soul, 'on account of the reference to the other.'
Not so, we reply, 'on account of the impossibility.' In the first place, the individual soul, which imagines itself to be limited by the internal organ and its other adjuncts, cannot be compared with the ether. And, in the second place, attributes such as freedom from evil, and the like, cannot be ascribed to a being which erroneously transfers to itself the attributes of its limiting adjuncts. This has already been set forth in the first Sûtra of the present adhikarana, and is again mentioned here in order to remove all doubt as to the soul being different from the highest Self. That the reference pointed out by the pûrvapakshin is not to the individual soul will, moreover, be shown in one of the next Sûtras (I, 3, 21).