The Vedanta Sutras of Badarayana, Commentary by Sankara (SBE38), tr. by George Thibaut  at sacred-texts.com
22. For (the clause 'Not so, not so') denies (of Brahman) the suchness which forms the topic of
discussion; and (the text) enounces something more than that.
We read, Bri. Up. II, 3, 'Two forms of Brahman there are indeed, the material and the immaterial, the mortal and the immortal, the solid and the fluid, sat and tya.' The text thereupon divides the five elements into two classes, predicates of the essence of that which is immaterial--which it calls purusha--saffron-colour, and so on, and then goes on to say, 'Now then the teaching by Not so, not so! For there is nothing else higher than this (if one says): It is not so.' Here we have to enquire what the object of the negative statement is. We do not observe any definite thing indicated by words such as 'this' or 'that;' we merely have the word 'so' in 'Not so, not so!' to which the word 'not' refers, and which on that account indicates something meant to be denied. Now we know that the word 'so' (iti) is used with reference to approximate things, in the same way as the particle 'evam' is used; compare, e.g. the sentence 'so (iti) indeed the teacher said' (where the 'so' refers to his immediately preceding speech). And, in our passage, the context points out what has to be considered as proximate, viz. the two cosmic forms of Brahman, and that Brahman itself to which the two forms belong. Hence there arises a doubt whether the phrase, 'Not so, not so!' negatives both Brahman and its two forms, or only either; and if the latter, whether it negatives Brahman and leaves its two forms, or if it negatives the two forms and leaves Brahman.--We suppose, the pûrvapakshin says, that the negative statement negatives Brahman as well as its two forms; both being suggested by the context. As the word 'not' is repeated twice, there are really two negative statements, of which the one negatives the cosmic form of Brahman, the other that which has form, i.e. Brahman itself. Or else we may suppose that Brahman alone is negatived. For as Brahman transcends all speech and thought, its existence is doubtful, and admits of being negatived; the plurality of cosmic forms on the other hand falls within the sphere of perception and the other means of right
knowledge, and can, therefore, not be negatived.--On this latter interpretation the repetition of 'not' must be considered as due to emphasis only.
To this we make the following reply. It is impossible that the phrase, 'Not so, not so!' should negative both, since that would imply the doctrine of a general Void. Whenever we deny something unreal, we do so with reference to something real; the unreal snake, e.g. is negatived with reference to the real rope. But this (denial of something unreal with reference to something real) is possible only if some entity is left. If everything is denied, no entity is left, and if no entity is left, the denial of some other entity which we may wish to undertake, becomes impossible, i.e. that latter entity becomes real and as such cannot be negatived.--Nor, in the second place, can Brahman be denied; for that would contradict the introductory phrase of the chapter, 'Shall I tell you Brahman?' (Bri. Up. II, 1, 1); would show disregard of the threat conveyed in Taitt. Up. II, 6, 'He who knows the Brahman as non-existing becomes himself non-existing;' would be opposed to definitive assertions such as 'By the words "He is" is he to be apprehended' (Ka. Up. II, 6, 13); and would involve a stultification of the entire Vedânta.--The phrase that Brahman transcends all speech and thought does certainly not mean to say that Brahman does not exist; for after the Vedânta-part of scripture has established at length the existence of Brahman--in such passages as 'He who knows Brahman obtains the highest;' 'Truth, knowledge, infinite is Brahman'--it cannot be supposed all at once to teach its non-existence. For, as the common saying is, 'Better than bathing it is not to touch dirt at all.' The passage, 'from whence all speech with the mind turns away unable to reach it' (Taitt. Up. II, 4), must, therefore, rather be viewed as intimating Brahman.
The passage of the Bri. Up. under discussion has, therefore, to be understood as follows. Brahman is that whose nature is permanent purity, intelligence, and freedom; it transcends speech and mind, does not fall within the category of 'object,' and constitutes the inward Self of all. Of this Brahman our text denies all plurality of forms; but
[paragraph continues] Brahman itself it leaves untouched. This the Sûtra expresses in the words, 'for it denies the suchness which forms the topic of discussion.' That means: The passage 'Not so,' &c., denies of Brahman the limited form, material as well as immaterial, which in the preceding part of the chapter is described at length with reference to the gods as well as the body, and also the second form which is produced by the first, is characterised by mental impressions, forms the essence of that which is immaterial, is denoted by the term purusha, rests on the subtle Self (liṅgâtman) and is described by means of comparisons with saffron-colour, &c., since the purusha, which is the essence of what is immaterial, does not itself possess colour perceivable by the eye. Now these forms of Brahman are by means of the word 'so' (iti), which always refers to something approximate brought into connexion with the negative particle 'not.' Brahman itself, on the other hand (apart from its forms), is, in the previous part of the chapter, mentioned not as in itself constituting the chief topic, but only in so far as it is qualified by its forms; this appears from the circumstance of Brahman being exhibited in the genitive case only ('These are two forms of Brahman'). Now, after the two forms have been set forth, there arises the desire of knowing that to which the two forms belong, and hence the text continues, 'Now then the teaching by means of "Not so, not so."' This passage, we conclude, conveys information regarding the nature of Brahman by denying the reality of the forms fictitiously attributed to it; for the phrase, 'Not so, not so!' negatives the whole aggregate of effects superimposed on Brahman. Effects we know to have no real existence, and they can therefore be negatived; not so, however, Brahman, which constitutes the necessary basis for all fictitious superimposition.--Nor must the question be asked here, how the sacred text, after having itself set forth the two forms of Brahman, can negative them in the end, contrary to the principle that not to touch dirt is better than bathing after having done so. For the text does not set forth the two forms of Brahman as something the truth of which is to be established, but merely mentions those two forms, which in
the sphere of ordinary thought are fictitiously attributed to Brahman, in order finally to negative them and establish thereby the true nature of the formless Brahman.
The double repetition of the negation may either serve the purpose of furnishing a special denial of the material as well as the immaterial form of Brahman; or the first 'Not so' may negative the aggregate of material elements, while the second denies the aggregate of mental impressions. Or else the repetition may be an emphatic one, intimating that whatever can be thought is not Brahman. This is, perhaps, the better explanation. For if a limited number of things are denied each individually, there still remains the desire to know whether something else may not be Brahman; an emphatic repetition of the denial on the other hand shows that the entire aggregate of objects is denied and that Brahman is the inward Self; whereby all further enquiry is checked.--The final conclusion, therefore, is, that the text negatives only the cosmic plurality fictitiously superimposed on Brahman, but leaves Brahman itself untouched.
The Sutra gives another argument establishing the same conclusion, 'and the text enounces something more than that,' i.e. more than the preceding negation. The words of the text meant are '(not) is there anything beyond.'--If the negation, 'Not so, not so!' were meant to negative all things whatever, and this terminated in absolute non-existence, the text could not even allude to 'anything beyond.'--The words of the text are to be connected as follows. After the clause, 'Not so, not so!' has given information about Brahman, the clause next following illustrates this teaching by saying: There is nothing beyond or separate from this Brahman; therefore Brahman is expressed by 'Not so, not so!' which latter words do not mean that Brahman itself does not exist. The implied meaning rather is that different from everything else there exists the 'non-negatived' Brahman.--The words of the text admit, however, of another interpretation also; for they may mean that there is no teaching of Brahman higher than that teaching which is implied in the negation of plurality expressed by 'Not so, not so!' On this latter interpretation
the words of the Sûtra, 'and the text enounces something more than that,' must be taken to refer to the name mentioned in the text, 'Then comes the name, the True of the True; the senses being the True and he the True of them.'--This again has a sense only if the previous negative clause denies everything but Brahman, not everything but absolute non-existence. For, if the latter were the case, what then could be called the True of the True?--We therefore decide that the clause, 'Not so, not so!' negatives not absolutely everything, but only everything but Brahman.