Everything in the manifest world — including the mind — is made of three qualities: sattva (clarity and light), rajas (activity and passion), and tamas (inertia and heaviness). Advaita uses the Sāṃkhya-Yoga guṇa framework to explain why some minds can hear the Upanishadic teaching and others cannot.
Look at the light early on a clear morning — cool, still, perfectly transparent. That quality is sattva: clarity, luminosity, the capacity for things to be seen accurately. Look at midday heat — intense, agitated, things distorting in the shimmer. That quality is rajas: activity, passion, the movement that both enables and disturbs. Look at deep winter night, heavy and still in a different way from the morning — sluggish, dense, resistant to movement. That quality is tamas: inertia, heaviness, the tendency to remain unchanged.
According to Sāṃkhya — the cosmological framework that Advaita and Yoga both draw on — all of manifest nature (prakṛti) is constituted by varying combinations of these three qualities (guṇas). Not just physical matter: the mind also is made of guṇas. A sattvic mind is clear, calm, and discriminating. A rajasic mind is restless, desiring, and agitated. A tamasic mind is dull, confused, and resistant to inquiry.
For Advaita, the guṇa framework explains why the same teaching lands differently in different students. The Mahāvākya heard by a sattvic mind can occasion the recognition directly. The same Mahāvākya heard by a rajasic mind becomes one more thing to want or argue about. Heard by a tamasic mind, it barely registers. This is not judgment. It is the observation that the vessel matters as much as the water.
Advaita's practical programme — karma, upāsanā, the sādhanacatuṣṭaya — is largely the programme of increasing sattva, reducing rajas and tamas, so the mind becomes transparent enough for the recognition to arise. Not because sattva is the self (it is not) but because a sattvic mind is the one that can be still enough to see through itself.
The Bhagavad Gītā's chapter 14 (Guṇatraya-vibhāga Yoga) is the canonical treatment of the three guṇas in relation to liberation. The chapter distinguishes the effects of each guṇa on the person: sattva binds through attachment to happiness and knowledge; rajas binds through attachment to action and its results; tamas binds through delusion and negligence. All three bind — even sattva. The Gītā's instruction: to go beyond all three guṇas (triguṇātīta).
Śaṅkara's commentary on Chapter 14 specifies what triguṇātīta means in Advaita terms: not the suppression or elimination of the guṇas (they continue to operate in the body-mind as long as prārabdha karma runs) but the recognition that the self is not any of the three guṇas, does not belong to any guṇa, and is the witness of the guṇas' operations. The jīvanmukta continues to have a body-mind constituted by guṇas — sattva predominates in the sage's mind, enabling the transparency of the recognition. But the sage does not identify with even sattva: even the quality of clarity is witnessed rather than identified with.
The guṇas present a philosophical tension for Advaita's doctrine of Brahman as nirguṇa (without guṇas). If all of manifest reality is constituted by guṇas, and the mind is part of manifest reality, then the mind that performs the inquiry and achieves the recognition is itself a guṇa-constituted instrument. How can a guṇa-constituted instrument recognise what is beyond all guṇas? Śaṅkara's answer: the mind (constituted by sattva-guṇa primarily) is an instrument, not the recogniser. The recognition is not performed by the mind — it is what happens when the sattvic mind becomes sufficiently transparent that the self-luminous Ātman/Brahman is no longer concealed by the mind's own operations. The mind does not see Brahman; the mind becomes still, and Brahman — which was always present — is no longer obscured. The guṇa-constituted instrument prepares the conditions; the recognition is Brahman's self-revelation.