Śaṃkarācārya attaches great importance to mind; in some of his interpretations, buddhi is also explained as manas only. According to him, antaḥkaraṇa is upādhi of Ātman and this upādhi is called by different names according to the context in which it is used. Again, antaḥkaraṇa is explained in two different ways. The first one, which is popular compromises of mind, intellect (buddhi), ego and citta. In the second one, ego or ahaṁkāra is replaced with vijñāna, which is explained as the act of distinguishing or discerning, understanding, comprehending, recognizing, intelligence, knowledge. Then, what is the difference between buddhi and vijñāna, as both are related to knowledge? Buddhi is explained as the power of forming and retaining conceptions and general notions; thus it corroborates the activities of the mind. But vijñāna is at higher level as it understands, comprehends and recognises.
The fourth and last aspect of antaḥkaraṇa is citta. What is citta? When Consciousness is limited and manifests in an individual, it is called citta. If Consciousness is considered as Brahman, then citta is the one which is bound by upādhi. Therefore, citta is also called individual consciousness or individual awareness. Citta is also referred cognitive apparatus, which is nothing but the mind. It is the cause for rekindling past impressions.
Self alone is illuminating, which is explained in various Upaniṣad-s. Unless Light is there within, we cannot see any objects. Let us see how this works.
Bṛhadāraṇayaka Upaniṣad (IV.iv.6) says “upon that immortal Light of all lights the gods meditate as longevity.” This means that gods meditate on this Supreme Light for their immortality.
Kaṭha Upaniṣad (II.ii.15) explains this further. “In the presence of Brahman, the sun does not shine, nor do the moon and stars, nor does lightning, let alone this fire. When Brahman shines, everything follows. By Its light, all these are lighted.”
“na tatra sūryo bhāti na candratārakaṃ
nemā vidyuto bhānti kutoyamagniḥ
tameva bhāntamanubhāti sarvaṃ
tasya bhāsā sarvamidaṃ vibhāti |”
Chāndogya Upaniṣad (VIII.iii.4) says, param joytiḥ upasampadyate which means attaining the highest light. The Upaniṣad says “Then, this person, who is the embodiment of happiness, emerging from the body and attaining the highest light, assumes his real nature. This is the Self.”
Buddhi, one of the constituents of antaḥkaraṇa stays very close to the ever illuminating Self. When we are saying Self, it refers to Kūṭastha in a human being. A jīva is a complex entity containing the Self, antaḥkaraṇa and the body (comprising of sensory organs). Next to buddhi is the mind; when we say mind, it includes citta and ego. The Light of the Self first enters buddhi and from buddhi to the mind and from the mind to the sensory organs and thus human cognition is evolved. Now, who is the seer now? Is it the Self or intellect, or mind or organs of action and perception? We know the Light for cognition is the Self within and without which, cognition is not possible. Self is the cause for cognition and obviously, Self is the seer and not others like antaḥkaraṇa or jñānendriya-s and karmendriya-s. Since Kūṭastha supports jīva, due to ignorance, we consider jīva as the seer and this is called innate ignorance.
We also know that Brahman is only a witness and does not partake in any of the activities of a being. That is why we call Brahman as the seer. Then, how and where the actions take place? All actions take place in māyā. When the Light of Brahman falls on māyā, Īśvara originates. Who is responsible for the origin of Īśvara? Māyā is responsible for the origin of Īśvara. Therefore, Īśvara is the ābhāsa of Brahman. Ābhāsa means reflection, semblance, having the mere appearance of a thing, etc. Brahman expands Itself as Īśvara to the visible world along with inherent māyā. Therefore, both Īśvara and māyā are the cause of this universe. Īśvara brings in qualitative change and māyā brings in various shapes and forms. In all such different shapes and forms, Kūṭastha remains as individual soul. Kūṭastha in turn is the expansion of Īśvara and Īśvara is the expansion of Brahman.