Sutra 2.4.10

10. And there is no objection on account of its not having an activity (karaṇa); for (Scripture) thus declares.

The karaṇa of the Sūtra means kriyā, action. The objection raised on the ground that the principal breath does not exercise any form of activity helpful to the soul, is without force, since as a matter of fact Scripture declares that there is such an activity, in so far as the vital breath supports the body with all its organs. For the text (Ch. Up. V, 1, 7 ff.) relates how on the successive departure of speech, and so on, the body and the other organs maintained their strength, while on the departure of the vital breath the body and all the organs at once became weak and powerless.—The conclusion therefore is that the breath, in its fivefold form of prāṇa, apāna, and so on, subserves the purposes of the individual soul, and thus occupies the position of an instrument, no less than the eye and the other organs.

But as those five forms of breath, viz. prāṇa, udāna, etc., have different names and functions they must be separate principles (and hence there is not one principal breath)! To this the next Sūtra replies.