The Vedanta Sutras of Badarayana, Commentary by Sankara (SBE38), tr. by George Thibaut  at sacred-texts.com
17. But on (the two roads) of knowledge and works, those two being under discussion.
In that place of the knowledge of the five fires, where the answer is expected to the question, 'Do you know why that world never becomes full?' the text runs as follows: 'On neither of these two ways are those small creatures continually returning, of whom it may be said, Live and die. Theirs is a third place. Therefore that world never becomes full.'
[paragraph continues] By the two ways mentioned in this passage we have to understand knowledge and works.--Why?--On account of their being the subjects under discussion. That means: knowledge and works are under discussion as the means for entering on the road of the gods and the road of the fathers. The clause, 'those who know this,' proclaims knowledge to be the means whereby to obtain the road of the gods; the clause, 'sacrifices, works of public utility, and alms,' proclaims works to be that by which we obtain the road of the fathers. Under the heading of these two paths there stands the subsequent passage, 'on neither of these two ways, &c.' To explain. Those who are neither entitled, through knowledge, to follow the road of the gods, nor, by works, to follow the road of the fathers, for those there is a third path on which they repeatedly return to the existence of small animals. For this reason also those who do not perform sacrifices. &c. do not reach the moon.--But why should they not first mount to the sphere of the moon and thence descending enter on the existence of small animals?--No, that would imply entire purposelessness of their mounting. Moreover, if all men when dying would reach the sphere of the moon, that world would be filled by the departed, and from that would result an answer contrary to the question (viz. 'why does not that world become full?'). For an answer is expected showing that that world does not become full.--Nor can we admit the explanation that the other world possibly does not become full because re-descent is admitted; since this is not stated by scripture. For it is true, indeed, that the not becoming full might be explained from their re-descending; but scripture actually explains it from the existence of a third place, 'Theirs is a third place; therefore that world never becomes full.' Hence the fact of the other world not becoming full must be explained from their not-ascending only. For, otherwise, the descent equally taking place in the case of those who do perform sacrifices, &c., it would follow that the statement of a third place is devoid of purpose.--The word 'but' (in the Sûtra) is meant to preclude the idea--arising from the passage of another sâkhâ (i.e. the Kaush. Up.)
[paragraph continues] --that all departed go to the moon. Under the circumstances the word 'all' which occurs in that passage has to be when as referring only to those qualified, so that the sense is 'all those who depart from this world properly qualified go to the moon.'--The next Sûtra is directed against the averment that all must go to the moon for the purpose obtaining a new body, in accordance with the definite statement of number ('in the fifth oblation &c.').