The Vedanta Sutras of Badarayana, Commentary by Sankara (SBE38), tr. by George Thibaut  at sacred-texts.com
31. On account of the appropriateness of the manifestation of that (connexion) which exists (potentially); like virile power.
As in ordinary life virile power and so on, existing potentially only in young children, and being then looked upon as non-existing, become manifest at the time of puberty--and do not originate at that time from previous non-existence, because in that case they might originate in eunuchs also--; so the connexion of the soul with the
buddhi exists potentially merely during deep sleep and the period of general retractation, and again becomes manifest at the time of waking and the time of creation.--This explanation is appropriate, because nothing can be assumed to spring up unless from something else; otherwise we should have to suppose that effects spring up without causes. That the rising from deep sleep is due to the existence of potential avidyâ, scripture also declares, 'Having become merged in the True they know not that they are merged in the True. Whatever these creatures are here, whether a lion or a wolf,' &c. (Kh. Up. VI, 9, 2; 3).--It is therefore a proved matter that the connexion of the soul with the buddhi and the other adjuncts lasts as long as the soul (in its samsâra-state).