The Vedanta Sutras of Badarayana, Commentary by Sankara (SBE38), tr. by George Thibaut  at sacred-texts.com
22. (On the part of the soul's descending from the
moon) there is entering into similarity of being (with ether and so on); as this (only) is possible.
It has been explained that the souls of those who perform sacrifices, &c., after having reached the moon dwell there as long as their works last and then re-descend with a remainder of their works. We now have to inquire into the mode of that descent. On this point scripture makes the following statement: 'They return again the way they came, to the ether, from the ether to the air. Then the sacrificer having become air becomes smoke, having become smoke he becomes mist, having become mist he becomes a cloud, having become a cloud he rains down.'--Here a doubt arises whether the descending souls pass over into a state of identity with ether, &c., or into a state of similarity.--The pûrvapakshin maintains that the state is one of identity, because this is directly stated by the text. Otherwise there would take place so-called indication (lakshanâ). Now whenever the doubt lies between a directly expressed and a merely indicated meaning the former is to be preferred. Thus the following words also, 'Having become air he becomes smoke,' &c., are appropriate only if the soul be understood to identify itself with them.--Hence it follows that the souls become identical with ether, &c.--To this we reply that they only pass into a state of similarity to ether, &c. When the body, consisting of water which the soul had assumed in the sphere of the moon for the purpose of enjoyment, dissolves at the time when that enjoyment comes to an end, then it becomes subtle like ether, passes thereupon into the power of the air, and then gets mixed with smoke, &c. This is the meaning of the clauses, 'They return as they came to the ether, from the ether to the air, &c.'--How is this known to be the meaning?--Because thus only it is possible. For it is not possible that one thing should become another in the literal sense of the word. If, moreover, the souls became identified with ether they could no longer descend through air, &c. And as connexion with the ether is, on account of its all-pervadingness, eternal, no other connexion (of the souls) with it can here be meant
but their entering into a state of similarity to it 1. And in cases where it is impossible to accept the literal meaning of the text it is quite proper to assume the meaning which is merely indicated.--For these reasons the souls' becoming ether, &c., has to be taken in the secondary sense of their passing into a state of similarity to ether, and so on.
128:1 It might be said that the relation to ether, &c., into which the souls enter, is the relation of conjunction (samyoga), not the relation of similarity. But as nothing can enter into the relation of samyoga with ether (everything being in eternal samyoga with it) we must assume that 'becoming ether' means 'becoming like ether,' and by parity of reasoning, that 'becoming air, &c.,' means 'becoming like air.'