The Vedanta Sutras, commentary by Sankaracharya (SBE34), tr. by George Thibaut  at sacred-texts.com
8. And (the pradhâna cannot be denoted by the word 'Self') because there is no statement of its having to be set aside.
If the pradhâna which is the Not-Self were denoted by the term 'Being' (Sat), and if the passage 'That is the Self, that art thou, O Svetaketu,' referred to the pradhâna; the teacher whose wish it is to impart instruction about the
true Brahman would subsequently declare that the pradhâna is to be set aside (and the true Brahman to be considered); for otherwise his pupil, having received the instruction about the pradhâna, might take his stand on the latter, looking upon it as the Non-Self. In ordinary life a man who wishes to point out to a friend the (small) star Arundhatî at first directs his attention to a big neighbouring star, saying 'that is Arundhatî,' although it is really not so; and thereupon he withdraws his first statement and points out the real Arundhatî. Analogously the teacher (if he intended to make his pupil understand the Self through the Non-Self) would in the end definitely state that the Self is not of the nature of the pradhâna. But no such statement is made; for the sixth Prapâthaka arrives at a conclusion based on the view that the Self is nothing but that which is (the Sat).
The word 'and' (in the Sûtra) is meant to notify that the contradiction of a previous statement (which would be implied in the rejected interpretation) is an additional reason for the rejection. Such a contradiction would result even if it were stated that the pradhâna is to be set aside. For in the beginning of the Prapâthaka it is intimated that through the knowledge of the cause everything becomes known. Compare the following consecutive sentences, 'Have you ever asked for that instruction by which we hear what cannot be heard, by which we perceive what cannot be perceived, by which we know what cannot be known? What is that instruction? As, my dear, by one clod of clay all that is made of clay is known, the modification (i.e. the effect) being a name merely which has its origin in speech, while the truth is that it is clay merely,' &c. Now if the term 'Sat' denoted the pradhâna, which is merely the cause of the aggregate of the objects of enjoyment, its knowledge, whether to be set aside or not to be set aside, could never lead to the knowledge of the aggregate of enjoyers (souls), because the latter is not an effect of the pradhâna. Therefore the pradhâna is not denoted by the term 'Sat.'--For this the Sûtrakâra gives a further reason.