The Vedanta Sutras of Badarayana, Commentary by Sankara (SBE38), tr. by George Thibaut  at sacred-texts.com
5. The one (word 'sprang') may be (taken in its secondary as well as in its primary sense), like the word 'Brahman.'
This Sûtra contains the reply to a doubt.--If we admit the opinion maintained hitherto, how can one and the same word 'sprang' ('from that Self sprang the ether') be used, in the same chapter, in its primary (real) meaning with regard to fire and so on, and in a secondary meaning with regard to ether?--The answer to this objection is that the one word 'sprang' may, according to the nature of the things to which it refers, be used in its primary as well as its secondary sense, just as the word 'Brahman' is used. For the one word 'Brahman' is, in the passage Taitt. Up. III, 2-6 ('Try to know Brahman by penance, for penance is Brahman'), used in a secondary sense with regard to food, &c., and in its primary sense with regard to bliss; and the same word Brahman is, in the way of figurative identification (bhakti), applied to penance, which is merely the means of knowing Brahman, and again directly to Brahman as the object of knowledge.--But how--to raise another question--can we, on the supposition of ether having no beginning, uphold the validity of the statement made in the clause 'one only, without a second?' For if ether is a second entity (co-existing with Brahman from eternity), it follows that Brahman has a second. And if so, how can it be said that when Brahman is known everything is known? (Kh. Up. VI, 1).--The word 'one,' the pûrvapakshin replies, may be used with reference to (the absence of) effects. As in ordinary life a person, who on a certain
day sees in a potter's house a lump of clay, a staff, a wheel and so on, and on the following day a number of finished vessels, might say, 'Yesterday there was only clay,' meaning thereby only that on the preceding day there were no things made of clay, not that there were no staff, wheel and the like; so the passage under discussion also is to be understood.--The term 'without a second' (does not exclude the existence from eternity of ether, but) excludes the existence of any other superintending being (but Brahman). While there is a superintending potter in addition to the material cause of the vessels, i.e. the clay, there is no other superintendent in addition to Brahman, the material cause of the world. Nor does the existence of ether as a second entity involve Brahman's being associated with a second (and therefore not being of a simple nature). For diversity is founded on difference of characteristic attributes, and before the origin (of the creation) no difference of attributes separating Brahman and ether exists; the two being mixed like water and milk, and having the common attributes of all-pervadingness, immateriality and so on. At the time of creation however a certain diversity of the two determines itself, Brahman putting forth energy in order to produce the world, while the ether remains immoveable.--And also from the passages quoted above--such as 'Brahman has the ether for its body'--it follows that the two are identical. Thence again it follows that through the knowledge of Brahman everything is known.--Moreover every effect, which is produced, is produced in such a way as not to be separated from ether in place as well as in time, and ether itself is non-separated in place and time from Brahman; hence, if there are known Brahman and its effects, the ether also is known. The case is similar to that of a few drops of water poured into a jug full of milk. Those drops are taken when the milk is taken; the taking of the drops does not constitute something additional to the taking of the milk. Analogously the ether, as being non-separate in place and time from Brahman and its effects, is comprised within Brahman, and consequently we have to understand the passages
about the origin of the ether in a secondary sense.--To this argumentation we make the following reply.