The Vedanta Sutras of Badarayana, Commentary by Sankara (SBE38), tr. by George Thibaut  at sacred-texts.com
37. There is exchange (of meditation), for the texts distinguish (two meditations); as in other cases.
The Aitareyins declare with reference to the person in the sun, 'What I am, that is he; what he is, that am I' (Ait. Âr. II, 2, 4, 6). And the Gâbâlas say, 'I am thou indeed, O reverend divinity, and thou art I indeed.'--The doubt here arises whether the reflection founded upon this text is to be a double one 'by means of exchange' (i.e. whether the soul is to be meditated upon as âditya and âditya as the soul), or a simple one (the soul only being meditated upon as âditya).
The pûrvapakshin maintains the latter view; for, he says, the text cannot possibly propose as matter of meditation anything but the oneness of the individual soul with the Lord. For if we assumed that two different forms of meditation are intended, viz. firstly the soul's being the Self of the Lord, and, secondly, the Lord's being the Self of the soul, the soul indeed would be exalted by the former meditation, but the Lord, at the same time, be lowered by the latter one. We therefore conclude that the meditation is to be of one kind only, and that the double form, in which the text exhibits it, merely aims at confirming the oneness of the Self.
To this we make the following reply. 'Exchange' is expressly recorded in the text for the purposes of meditation, just as other qualities (of the Self), such as its being the Self of all, &c., are recorded for the same purpose. For both texts make the distinctive double enunciation, 'I am thou,' and 'Thou art I.' Now this double enunciation has a sense only if a twofold meditation is to be based upon it; otherwise it would be devoid of meaning, since one statement would be all that is required.--But has not the pûrvapakshin urged above that this your explanation involves a lowering of the Lord, who is thereby represented as having the transmigrating soul for his Self?--Never mind, we reply; even in that way only the unity of the Self is meditated upon.--But does your explanation
then not come to that of the pûrvapakshin, viz. that the double statement is merely meant to confirm the oneness of the Self?--We do not, our reply is, deny that the text confirms the oneness of the Self; we only want to prove that, on the ground of the text as it stands, a twofold meditation has to be admitted, not a simple one. That this virtually confirms the unity of the Self we admit; just as the instruction about (the Lord's) possessing such qualities as having only true wishes, and so on--which instruction is given for the purpose of meditation--at the same time proves the existence of a Lord endowed with such qualities.--Hence the double relation enounced in the text has to be meditated upon, and is to be transferred to other vidyâs also which treat of the same subject.