Sutra 2.4.9

9. But like the eye and the rest, on account of being taught with them, and for other reasons.

Breath is not an element, but like sight and the rest, a special instrument of the soul. This appears from the fact that the texts mention it together with the recognised organs of the soul, the eye, and so on; so e.g. in the colloquy of the prāṇas. And such common mention is suitable in the case of such things only as belong to one class.—The 'and for other reasons' of the Sūtra refers to the circumstance of the principal breath being specially mentioned among the organs comprised under the term 'prāṇa'; cp. 'that principal breath' (Ch. Up. I, 2, 7); 'that central breath' (Bṛ. Up. I, 5, 21).—But if the chief breath is, like the eye and the other organs, an instrument of the soul, there must be some special form of activity through which it assists the soul, as the eye e.g. assists the soul by seeing. But no such activity is perceived, and the breath cannot therefore be put in the same category as the organs of sensation and action!—To this objection the next Sūtra replies.