The Vedanta Sutras of Badarayana, Commentary by Sankara (SBE38), tr. by George Thibaut [1896] at sacred-texts.com
18. For this very reason (the individual soul is) intelligent.
Owing to the conflicting views of the philosophical schools there arises a doubt whether, as the followers of Kanâda think, the soul is in itself non-intelligent, so that its intelligence is merely adventitious; or if. as the Sâṅkhyas think, eternal intelligence constitutes its very nature.
The pûrvapakshin maintains that the intelligence of the Self is adventitious, and is produced by the conjunction of the Self with the mind (manas), just as, for instance, the quality of redness is produced in a jar by the conjunction of the jar with fire. For if the soul were of eternal (essential) intelligence, it would remain intelligent in the states of deep sleep, swoon, and possession, while as a matter of fact, men when waking from sleep and so on declare in reply to questions addressed to them that they were not conscious of anything. Men in their ordinary state, on the other hand, are seen to be (actively) intelligent. Hence, as intelligence
is clearly intermittent, we conclude that the Self's intelligence is adventitious only.
To this reply that the soul is of eternal intelligence, for that very reason that it is not a product but nothing else but the unmodified highest Brahman which, owing to the contact with its limiting adjuncts, appears as individual soul. That intelligence constitutes the essential nature of the highest Brahman, we know from scriptural passages such as 'Brahman is knowledge and bliss' (Bri. Up. III, 9, 28, 7); 'Brahman is true, knowledge, infinite' (Taitt. Up. II, 1); 'Having neither inside nor outside, but being altogether a mass of knowledge' (Bri. Up. IV, 5, 13). Now, if the individual soul is nothing but that highest Brahman, then eternal intelligence constitutes the soul's essential nature also, just as light and heat constitute the nature of fire. In the chapter treating of that which consists of knowledge, there are, moreover, passages (directly declaring that the individual soul is of the nature of self- luminous intelligence), 'He not asleep himself looks down upon the sleeping (senses)' (Bri. Up. IV, 3, 11); 'That person is self-illuminated' (Bri. Up. IV, 3, 14); 'For there is no intermission of the knowing of the knower' (Bri. Up. IV, 3, 30). That the soul's nature is intelligence, follows moreover from the passage (Kh. Up. VIII, 12, 4) where it is represented as connected with knowledge through all sense-organs, 'He who knows let me smell this, he is the Self,' &c. &c.--From the soul's essential nature being intelligence it does not follow that the senses are useless; for they serve the purpose of determining the special object of each sense, such as smell and so on. This is expressly declared by scripture, 'Smell is for the purpose of perceiving odour' (Kh. Up. VIII, 12, 4).--The objection that sleeping persons are not conscious of anything is refuted by scripture, where we read concerning a man lying in deep sleep, 'And when there he does not see, yet he is seeing through he does not see. For there is no intermission of the seeing of the seer, because it cannot perish. But there is then no second, nothing else different from him that he could see' (Bri. Up. IV, 3, 23). That means:
[paragraph continues] The absence of actual intelligising is due to the absence of objects, not to the absence of intelligence; just as the light pervading space is not apparent owing to the absence of things to be illuminated, not to the absence of its own nature.--The reasoning of the Vaiseshikas and others is, as contradicting scripture, merely fallacious, and we therefore decide that eternal intelligence is the essential nature of the soul.