Śaṅkaraon Brahma Sūtra 3.2.23

If that highest Brahman which is different from the world that is negatived in the passage discussed above really exists, why then is it not apprehended ?—Because, the Sutrakara replies, it is unevolved, not to be apprehended by the senses ; for it is the witness of whatever is apprehended (i.e. the subject in all apprehension). Thus 5ruti says, ‘ He is not apprehended by the eye, nor by speech, nor by the other senses, not by penance or good works’ (Mu. Up. Ill, 1, 8) ; ‘That Self is to be described by No, no ! He is incomprehensible, for he cannot be comprehended’ (Bri. Up. Ill, 9, 26) ; ‘That which cannot be seen nor apprehended’ (Mu. Up. I, 1, 6) ; ‘When in that which is invisible, incorporeal, undefined, unsupported ’ &c. (Taitt. Up. II, 7). Similar statements are made in Smr^ti-passages; so e. g. ‘ He is called unevolved, not to be fathomed by thought, unchangeable.’