The Vedanta Sutras of Badarayana, Commentary by Sankara (SBE38), tr. by George Thibaut  at sacred-texts.com
3. That mind (is merged) in breath, owing to the subsequent clause.
It has been shown that the passage, 'Speech is merged in mind,' means a merging of the function only.--A doubt here arises whether the subsequent clause, 'mind in breath,' also means to intimate a merging of the function only or of that to which the function belongs.--The pûrvapakshin maintains the latter alternative. For that, he says, agrees with scripture, and moreover breath may be viewed as the causal substance of mind. For scripture--'Mind is made of earth, breath of water' (Kh. Up. VI, 6, 5)--states that mind comes from earth and breath from water, and scripture further states that 'Water sent forth earth' (Kh. Up. VI, 2, 4). When mind therefore is merged in breath, it is the same as earth being merged in water; for mind is earth and breath is water, causal substance and effect being non-different.
To this we reply as follows. 'The subsequent clause' intimates that the mind, after having absorbed within itself the functions of the outer senses, is merged in breath only in the way of its function being so merged. For we observe in the case of persons lying in deep sleep or about to die that, while the function of breath persists, the functions of the mind are stopped. Nor is the mind capable of being itself merged in breath, since breath does not constitute its causal substance.--But it has been shown above that breath is the causal substance of mind!--This is not valid,
we reply. For the relation of causality, made out in such an indirect way, does not suffice to show that mind is really merged in breath. Were it so, then mind would also be merged in earth, earth in water, breath in water. Nor is there, on the alternative contemplated, any proof of mind having originated from that water which had passed over into breath.--Mind cannot therefore, in itself, be merged in breath. And that the scriptural statement is satisfied by a mere merging of the function--the function and that to which the function belongs being viewed as identical--has been shown already under the preceding Sûtra.