Brahma Knowledge, by L. D. Barnett, , at sacred-texts.com
§ 13. Brahma is Incogitable.—Brahma or Ātmā, being an absolute Reality, the supreme Thing-in-Itself, is therefore inconceivable by the reason, and only capable of being comprehended by an inspired intuition. The Self, whether universal or individual, is a single subject of thought, and so cannot be an object of thought; and it is by nature absolute, above all conditions of finite determination. This negative conception is most forcibly expressed by the famous formula ascribed to Yājnavalkya, neti neti, "not so, not so," a denial of all possible predicates to the Self (B.A. IV. ii. 4, IV. 22, v. 15, etc.), and also by frequently describing Brahma or Ātmā in contradictory terms as limitless, infinitely extended and yet immeasurably limited in space and time, and denying to it all activity as cause or effect. See also B.A. II. iv. 14, III. iv. 2, vii. 23, viii. 11, Ch. III. xiv. 2, VII. xxiv. 1, Kena. III., XI., Kaṭh. VI. 12 f. Śankara (on Brahma-sūtra II. iii. 29) explains these contradictions in the description of the size of the Self by asserting that the mention of it as immeasurably small refers to its condition as empiric soul, when it is limited by "determinations" (§ 12) and is conceived under the attributes of the "determination" of intelligence, buddhi (§ 18).
Śankara (on Brahma-sūtra III. ii. 17) tells a tale of a sage who, on being asked to teach the doctrine of Brahma, remained silent, and on the request
being thrice repeated said simply, "I have told you, but you understand not; this Self is still" (cf. Ch. III. xiv. 1).