The Vedanta Sutras, commentary by Sankaracharya (SBE34), tr. by George Thibaut  at sacred-texts.com
15. On account of the connexion (with passages treating of Brahman, the passages speaking of the Non-being do not intimate absolute Non-existence).
The passage 'Non-being indeed was this in the beginning' (Taitt. Up. II, 7) does not declare that the cause of the world is the absolutely Non-existent which is devoid of all Selfhood. For in the preceding sections of the Upanishad Brahman is distinctly denied to be the Non-existing, and is defined to be that which is ('He who knows the Brahman as non-existing becomes himself non-existing.
[paragraph continues] He who knows the Brahman as existing him we know himself as existing'); it is further, by means of the series of sheaths, viz. the sheath of food, &c., represented as the inner Self of everything. This same Brahman is again referred to in the clause, 'He wished, may I be many;' is declared to have originated the entire creation; and is finally referred to in the clause, 'Therefore the wise call it the true.' Thereupon the text goes on to say, with reference to what has all along been the topic of discussion, 'On this there is also this sloka, Non-being indeed was this in the beginning,' &c.--If here the term 'Non-being' denoted the absolutely Non-existent, the whole context would be broken; for while ostensibly referring to one matter the passage would in reality treat of a second altogether different matter. We have therefore to conclude that, while the term 'Being' ordinarily denotes that which is differentiated by names and forms, the term 'Non-being' denotes the same substance previous to its differentiation, i.e. that Brahman is, in a secondary sense of the word, called Non-being, previously to the origination of the world. The same interpretation has to be applied to the passage 'Non-being this was in the beginning' (Kh. Up. III, 19, 1); for that passage also is connected with another passage which runs, 'It became being;' whence it is evident that the 'Non-being' of the former passage cannot mean absolute Non-existence. And in the passage, 'Others say, Non-being this was in the beginning' (Kh. Up. VI, 2, 1), the reference to the opinion of 'others' does not mean that the doctrine referred, to (according to which the world was originally absolutely non-existent) is propounded somewhere in the Veda; for option is possible in the case of actions but not in the case of substances. The passage has therefore to be looked upon as a refutation of the tenet of primitive absolute non-existence as fancifully propounded by some teachers of inferior intelligence; a refutation undertaken for the purpose of strengthening the doctrine that this world has sprung from that which is.--The following passage again, 'Now this was then undeveloped,' &c. (Bri. Up. I, 4, 7), does not by any means assert that the evolution of
the world took place without a ruler; as we conclude from the circumstance of its being connected with another passage in which the ruler is represented as entering into the evolved world of effects, 'He entered thither to the very tips of the finger-nails' &c. If it were supposed that the evolution of the world takes place without a ruler, to whom could the subsequent pronoun 'he' refer (in the passage last quoted) which manifestly is to be connected with something previously intimated? And as Scripture declares that the Self, after having entered into the body, is of the nature of intelligence ('when seeing, eye by name; when hearing, ear by name; when thinking, mind by name'), it follows that it is intelligent at the time of its entering also.--We, moreover, must assume that the world was evolved at the beginning of the creation in the same way as it is at present seen to develop itself by names and forms, viz. under the rulership of an intelligent creator; for we have no right to make assumptions contrary to what is at present actually observed. Another scriptural passage also declares that the evolution of the world took place under the superintendence of a ruler, 'Let me now enter these beings with this living Self, and let me then evolve names and forms' (Kh. Up. VI, 3, 2). The intransitive expression 'It developed itself' (vyâkriyata; it became developed) is to be viewed as having reference to the ease with which the real agent, viz. the Lord, brought about that evolution. Analogously it is said, for instance, that 'the cornfield reaps itself' (i.e. is reaped with the greatest ease), although there is the reaper sufficient (to account for the work being done).--Or else we may look on the form vyâkriyata as having reference to a necessarily implied agent; as is the case in such phrases as 'the village is being approached' (where we necessarily have to supply 'by Devadatta or somebody else').