The Vedanta Sutras, commentary by Sankaracharya (SBE34), tr. by George Thibaut  at sacred-texts.com
5. Nor (can it be said that the pradhâna modifies itself spontaneously) like grass, &c. (which turn into milk); for (milk) does not exist elsewhere (but in the female animal).
Let this be (the Sânkhya resumes). Just as grass, herbs, water, &c. independently of any other instrumental cause transform themselves, by their own nature, into milk; so, we assume, the pradhâna also transforms itself into the great principle, and so on. And, if you ask how we know that grass transforms itself independently of any instrumental cause; we reply, 'Because no such cause is observed.' For if we did perceive some such cause, we certainly should apply it to grass, &c. according to our liking, and thereby produce milk. But as a matter of fact we do no such thing. Hence the transformation of grass and the like must be considered to be due to its own nature merely; and we may infer therefrom that the transformation of the pradhâna is of the same kind.
To this we make the following reply.--The transformation of the pradhâna might be ascribed to its own nature merely if we really could admit that grass modifies itself in the manner stated by you; but we are unable to admit that, since another instrumental cause is observed. How? 'Because it does not exist elsewhere.' For grass becomes milk only when it is eaten by a cow or some other female animal, not if it is left either uneaten or is eaten by a bull. If the transformation had no special cause, grass would become milk even on other conditions than that of entering a cow's body. Nor would the circumstance of men not being able to produce milk according to their liking prove that there is no instrumental cause; for while some effects can be produced by men, others result from divine action only 1. The fact, however, is that men also are able, by
applying a means in their power, to produce milk from grass and herbs; for when they wish to procure a more abundant supply of milk they feed the cow more plentifully and thus obtain more milk from her.--For these reasons the spontaneous modification of the pradhâna cannot be proved from the instance of grass and the like.
371:1 It might be held that for the transformation of grass into milk no other cause is required than the digestive heat of the cow's p. 372 body; but a reflecting person will acknowledge that there also the omniscient Lord is active. Bhâ.