The Vedanta Sutras, commentary by Sankaracharya (SBE34), tr. by George Thibaut [1890] at sacred-texts.com
22. On account of the acting after (i.e. the shining after), (that after which sun, moon, &c. are said to shine is the highest Self), and (because by the light) of him (all this is said to be lighted).
We read (Mu. Up. II, 2, 10, and Ka. Up. V, 15), 'The sun does not shine there, nor the moon and the stars, nor these lightnings, much less this fire. After him when he shines everything shines; by the light of him all this is lighted.' The question here arises whether he 'after whom when he shines everything shines, and by whose light all this is lighted,' is some luminous substance, or the highest Self (prâgña âtman).
A luminous substance, the pûrvapakshin maintains.--Why?--Because the passage denies the shining only of such luminous bodies as the sun and the like. It is known (from every-day experience) that luminous bodies such as the moon and the stars do not shine at daytime when the sun, which is itself a luminous body, is shining. Hence we infer that that thing on account of which all this, including the moon, the stars, and the sun himself, does not
shine is likewise a thing of light. The 'shining after' also is possible only if there is a luminous body already, for we know from experience that 'acting after' (imitation) of any kind takes place only when there are more than one agent of similar nature; one man, for instance, walks after another man who walks himself. Therefore we consider it settled that the passage refers to some luminous body.
To this we reply that the highest Self only can be meant.--Why?--On account of the acting after. The shining after mentioned in the passage, 'After him when he shines everything shines,' is possible only if the prâgña Self, i.e. the highest Self, is understood. Of that prâgña Self another scriptural passage says, 'His form is light, his thoughts are true' (Kh. Up. III, 14, 2). On the other hand, it is not by any means known that the sun, &c. shines after some other luminous body. Moreover, on account of the equality of nature of all luminous bodies such as the sun and the like, there is no need for them of any other luminous body after which they should shine; for we see that a lamp, for instance, does not 'shine after' another lamp. Nor is there any such absolute rule (as the pûrvapakshin asserted) that acting after is observed only among things of similar nature. It is rather observed among things of dissimilar nature also; for a red-hot iron ball acts after, i.e. burns after the burning fire, and the dust of the ground blows (is blown) after the blowing wind.--The clause 'on account of the acting after' (which forms part of the Sûtra) points to the shining after (mentioned in the scriptural sloka under discussion); the clause 'and of him' points to the fourth pâda of the same sloka. The meaning of this latter clause is that the cause assigned for the light of the sun, &c. (in the passage 'by the light of him everything is lighted') intimates the prâgña Self. For of that Self Scripture says, 'Him the gods worship as the light of lights, as immortal time' (Bri. Up. IV, 4, 16). That, on the other hand, the light of the sun, the moon, &c, should shine by some other (physical) light is, in the first place, not known; and, in the second place, absurd
as one (physical) light is counteracted by another.--Or else the cause assigned for the shining does not apply only to the sun and the other bodies mentioned in the sloka; but the meaning (of the last pâda) rather is--as we may conclude from the comprehensive statement 'all this'--that the manifestation of this entire world consisting of names and forms, acts, agents and fruits (of action) has for its cause the existence of the light of Brahman; just as the existence of the light of the sun is the cause of the manifestation of all form and colour.--Moreover, the text shows by means of the word 'there' ('the sun does not shine there,' &c.) that the passage is to be connected with the general topic, and that topic is Brahman as appears from Mu. Up. II, 2, 5, 'In whom the heaven, the earth, and the sky are woven,' &c. The same appears from a passage subsequent (on the one just quoted and immediately preceding the passage under discussion). 'In the highest golden sheath there is the Brahman without passion and without parts; that is pure, that is the light of lights, that is it which they know who know the Self.' This passage giving rise to the question, 'How is it the light of lights?' there is occasion for the reply given in 'The sun does not shine there,' &c.--In refutation of the assertion that the shining of luminous bodies such as the sun and the moon can be denied only in case of there being another luminous body--as, for instance, the light of the moon and the stars is denied only when the sun is shining--we point out that it has been shown that he (the Self) only can be the luminous being referred to, nothing else. And it is quite possible to deny the shining of sun, moon, and so on with regard to Brahman; for whatever is perceived is perceived by the light of Brahman only so that sun, moon, &c. can be said to shine in it; while Brahman as self-luminous is not perceived by means of any other light. Brahman manifests everything else, but is not manifested by anything else; according to such scriptural passages as, 'By the Self alone as his light man sits,' &c. (Bri. Up. IV, 3, 6), and 'He is incomprehensible, for he cannot be comprehended '(Bri. Up. IV, 2, 4).