The Vedanta Sutras, commentary by Sankaracharya (SBE34), tr. by George Thibaut  at sacred-texts.com
15. (The small ether is Brahman) on account of
the action of going (into Brahman) and of the word (brahmaloka); for thus it is seen (i. e. that the individual souls go into Brahman is seen elsewhere in Scripture); and (this going of the souls into Brahman constitutes) an inferential sign (by means of which we may properly interpret the word 'brahmaloka').
It has been declared (in the preceding Sûtra) that the small (ether) is the highest Lord, on account of the reasons contained in the subsequent passages. These subsequent reasons are now set forth.--For this reason also the small (ether) can be the highest Lord only, because the passage complementary to the passage concerning the small (ether) contains a mention of going and a word, both of which intimate the highest Lord. In the first place, we read (Kh. Up. VIII, 3, 2), 'All these creatures, day after day going into that Brahma-world, do not discover it.' This passage which refers back, by means of the word 'Brahma-world,' to the small ether which forms the general subject-matter, speaks of the going to it of the creatures, i. e. the individual souls, wherefrom we conclude that the small (ether) is Brahman. For this going of the individual souls into Brahman, which takes place day after day in the state of deep sleep, is seen, i.e. is met with in another scriptural passage, viz. Kh. Up. VI, 8, 1, 'He becomes united with the True,' &c. In ordinary life also we say of a man who lies in deep sleep, 'he has become Brahman,' 'he is gone into the state of Brahman.'--In the second place, the word 'Brahma-world,' which is here applied to the small (ether) under discussion, excludes all thought of the individual soul or the elemental ether, and thus gives us to understand that the small (ether) is Brahman.--But could not the word 'Brahma-world' convey as well the idea of the world of him whose throne is the lotus 1?--It might do so indeed, if we explained the compound 'Brahma-world' as 'the world of Brahman.' But if we explain it on the ground of the coordination of both members of the compound--so that
[paragraph continues] 'Brahma-world' denotes that world which is Brahman--then it conveys the idea of the highest Brahman only.--And that daily going (of the souls) into Brahman (mentioned above) is, moreover, an inferential sign for explaining the compound 'Brahma-world,' on the ground of the co-ordination of its two constituent members. For it would be impossible to assume that all those creatures daily go into the world of the effected (lower) Brahman; which world is commonly called the Satyaloka, i. e. the world of the True.
180:1 I. e. Brahmâ, the lower Brahman.