The Vedanta Sutras, commentary by Sankaracharya (SBE34), tr. by George Thibaut  at sacred-texts.com
40. The light (is Brahman), on account of that (Brahman) being seen (in the scriptural passage).
We read in Scripture, 'Thus does that serene being, arising from this body, appear in its own form as soon as it has approached the highest light' (Kh. Up. VIII, 12, 3). Here the doubt arises whether the word 'light' denotes the (physical) light, which is the object of sight and dispels darkness, or the highest Brahman.
The pûrvapakshin maintains that the word 'light' denotes the well-known (physical) light, because that is the conventional sense of the word. For while it is to be admitted that in another passage, discussed under I, 1, 24, the word 'light' does, owing to the general topic of the chapter, divest itself of its ordinary meaning and denote Brahman, there is in our passage no similar reason for setting the ordinary meaning aside. Moreover, it is stated in the chapter treating of the nâdîs of the body, that a man going to final release reaches the sun ('When he departs from this body then he departs upwards by those very rays;' Kh. Up. VIII, 6, 5). Hence we conclude that the word 'light' denotes, in our passage, the ordinary light.
To this we make the following reply.--The word 'light' can denote the highest Brahman only, on account of that being seen. We see that in the whole chapter Brahman is carried on as the topic of discussion. For the Self, which is free from sin, &c. is introduced as the general subject-matter in VIII, 7, 1 ('the Self which is free from sin '); it is thereupon set forth as that which is to be searched out and to be understood (VIII, 7, 1); it is carried on by means of the clauses, 'I shall explain that further to you' (VIII, 9, 3 ff.); after that freedom from body is said to belong to it, because it is one with light ('when he is free from the body then neither pleasure nor pain touches him,' VIII, 12, 1)--and freedom from body is not possible outside Brahman--and it is finally qualified as 'the highest light, the highest person' (VIII, 12, 3).--Against the statement, made by the pûrvapakshin, that Scripture speaks of a man going to release as reaching the sun, we remark, that the release there referred to is not the ultimate one, since it is said to be connected with going and departing upwards. That the ultimate release has nothing to do with going and departing upwards we shall show later on.