The Vedanta Sutras of Badarayana, Commentary by Sankara (SBE38), tr. by George Thibaut  at sacred-texts.com
13. And because scripture declares that.
The text, 'Going upwards by that he reaches immortality,' declares that immortality is reached by going. But immortality is possible only in the highest Brahman, not in the effected one, because the latter is transitory. So scripture says, 'Where one sees something else, that is little, that is mortal' (Kh. Up. VII, 24, 1). According to the text of the Katha-upanishad also the going of the soul is towards the highest Brahman; for after the highest Brahman has been introduced there as general subject-matter--in the passage, 'That which thou seest,' &c., I, 2, 14, no other kind of knowledge is taken up later on.