"Not this, not this", from the Brhad-aranyaka Upanishad, which many people (such as Sankaracharya and his school) have wrongly interpreted to mean that the Supreme Brahman

is devoid of all attributes.

Here are Ramanuja's comments, written in the course of his commentary on Brahma-sutras 3.2.21:

   21. For the text denies the previously declared so-muchness; and declares more than that.

   It is impossible to understand the text `not so, not so' as negating those distinctions of Brahman which had been stated previously.  If the text meant that, it would be mere idle talk. For none but a person not in his right mind would first teach that all the things mentioned in the earlier part of the section are distinctive attributes of Brahman--as which they are not known by any means of proof--and thereupon deliberately negates

   his own teaching.  Although among the things mentioned there are some which, in themselves, are known through other means of proof, yet they are not thus known to be modes of Brahman, and others again are known neither in themselves nor as modes of

   Brahman.

   The text therefore cannot merely refer to them as things otherwise known, but gives fundamental instruction about them. Hence, the later passage cannot be meant as a sheer negation, but must be taken as denying the previously described 'so-muchness' of Brahman; i.e., the passage denies that limited nature of Brahman which would result from Brahman being viewed as distinguished by the previous stated attributes only.  The word

   "so" refers to that limited nature, and the phrase "not so" therefore means that Brahman is not distinguished by the previously stated modes "only".

   This interpretation is further confirmed by the fact that after the negative phrase, further qualities of Brahman are declared by the text: "For there is not anything higher than this -- not so. Then comes the name, the True of the true; for the prANas are the true, and the is the True of them."

   This means that other than Brahman which is expressed by the phrase `not so' there is nothing higher, i.e., there is nothing more exalted than Brahman either in essential nature or in qualities.  And of that Brahman the name is `True of the True'. This name is explained in the next clause, `for the prANas,' etc. The term prANas here denotes the individual souls, so called because the prANas accompany them.  They are the 'true' because they do not, like the elements, undergo changes implying an alteration of their essential nature.  And the highest Self is the 'True of the true' because while the souls undergo, in accordance with their karma, contractions and expansions of  knowledge, the highest Self which is free from all sin knows of no such alternations.  He is therefore more eminently 'true' than they are.

   As thus, the complementary passage declares Brahman to be connected with certain qualities, the clause 'not so, not so' (to which that passage is complementary) cannot deny that Brahman possesses distinctive attributes, but only denies that Brahman's nature is confined to the attributes previously stated.