The Vedanta Sutras, commentary by Sankaracharya (SBE34), tr. by George Thibaut  at sacred-texts.com
24. And in the case of space also (the doctrine of its being a non-entity is untenable) on account of its not differing (from the two other kinds of non-entity).
We have shown so far that of the triad declared by the Bauddhas to be devoid of all positive characteristics, and therefore non-definable, two (viz. prati-samkhyâvirodha and aprati°) cannot be shown to be such; we now proceed to show the same with regard to space (ether, âkâsa).
With regard to space also it cannot be maintained that it is non-definable, since substantiality can be established in the case of space no less than in the case of the two so-called non-entities treated of in the preceding Sutras. That space is a real thing follows in the first place from certain scriptural passages, such as 'space sprang from the Self.'--To those, again, who (like the Bauddhas) disagree with us as to the authoritativeness of Scripture we point out that the real existence of space is to be inferred from the quality of sound, since we observe that earth and other real things are the abodes of smell and the other qualities.--Moreover, if you declare that space is nothing but the absence in general of any covering (occupying) body, it would follow that while one bird is flying--whereby space is occupied--there would be no room for a second bird wanting to fly at the same time. And if you should reply that the second bird may fly there where there is absence of a covering body, we point out that that something by which the absence of covering bodies is distinguished must be a positive entity, viz. space in our sense, and not the mere non-existence of covering bodies 1.--Moreover, the Bauddha places himself, by his view of space, in opposition to other parts of his system. For we find, in the Bauddha Scriptures, a series of questions and answers (beginning, 'On what, O reverend Sir, is the earth founded?'), in which the following
question occurs, 'On what is the air founded?' to which it is replied that the air is founded on space (ether). Now it is clear that this statement is appropriate only on the supposition of space being a positive entity, not a mere negation.--Further, there is a self-contradiction in the Bauddha statements regarding all the three kinds of negative entities, it being said, on the one hand, that they are not positively definable, and, on the other hand, that they are eternal. Of what is not real neither eternity nor non-eternity can be predicated, since the distinction of subjects and predicates of attribution is founded entirely on real things. Anything with regard to which that distinction holds good we conclude to be a real thing, such as jars and the like are, not a mere undefinable negation.
412:1 What does enable us to declare that there is âvaranâbhâva in one place and not in another? Space; which therefore is something real.