Adi Shankara

On brahman, which is neither perceived nor inferred from the world, the only valid source is revelation (śruti), the word of the Upaniṣads: sense experience gives only the manifold, and reasoning alone cannot establish the absolute. But Śaṅkara is no fideist: reason (yukti) is indispensable to interpret Scripture, resolve its apparent contradictions and refute rival schools, and the teaching saves only when it becomes direct experience of the self. The primary material is thus not empirical but scriptural and rational, which places him on the side of reason without making him a pure rationalist, the authority of revealed testimony remaining first.

Commentary on the Brahma Sūtras, I.1.3-4 · Upadeśasāhasrī

If bondage is only an illusory superimposition (māyā), no act or rite can undo it: only the knowledge that discriminates the real from the apparent dissolves illusion, as light dispels the snake one thought one saw in the rope. Liberation is not produced by knowledge: it is that very knowledge, recognizing that the self was never bound.

Upadeśasāhasrī · Commentaries on the Upaniṣads

The true self, the ātman, is neither the body, nor the mind, nor the changing sequence of psychic states: it is pure consciousness, the permanent ever-present witness that every experience presupposes. It cannot be denied without being confirmed, for the one who would deny it would still be it. Śaṅkara thus takes the exact opposite of the bundle thesis: where one would find only a flux of impressions without a bearer, as Hume would hold, or before him the Buddhist not-self (anātman), he maintains an irreducible subject. Yet this permanent core is not one individual soul among others: it is identical in all and identical to brahman, which radicalizes substantialism while depersonalizing it.

Upadeśasāhasrī, II (prose part), on "thou art that" · Commentary on the Bṛhadāraṇyaka Upaniṣad

The sovereign good is liberation (mokṣa), understood as the recognition of the identity of the self and the absolute: not a pleasure, not even a virtue for its own sake, but the achievement of what one already is. This liberation is described as bliss (ānanda), a beatitude that is the very nature of brahman: we are close to a eudaimonism, the sovereign good being the realized fullness of being. A perfectionist strand remains, since the path demands discipline, renunciation and purification of the mind, but these means do not create liberation: they remove what was veiling it.

Commentary on the Taittirīya Upaniṣad (on ānanda) · Upadeśasāhasrī

Desire for objects binds one to saṃsāra, the cycle of rebirths: it arises from the ignorance that takes the self for the body and chases after what it lacks. Among the conditions of knowledge, Śaṅkara places renunciation (vairāgya) of the fruits of action, here and hereafter, and the ardent quest for liberation alone. The slope is clearly that of detachment: not because pleasure would be culpable, but because every desire turned toward the manifold misses the one real and the only stable good. A sorting remains in the background, the desire for liberation (mumukṣutva) being itself cultivated, but as a desire meant to extinguish once its end is reached.

Commentary on the Brahma Sūtras, I.1.1 (the fourfold prerequisite) · Commentary on the Bhagavad Gītā

Against the ritualists (Mīmāṃsā), for whom the performance of prescribed acts is the way, Śaṅkara holds that action does not liberate: it stays within the field of ignorance and yields only transient fruits. Only the turn inward, the inquiry into "who am I?" and the knowledge of the self, leads to liberation. The discipline consists of hearing the teaching (śravaṇa), reflection (manana) and meditation (nididhyāsana): a patient examination turned toward the witness of all experience, the opposite of a primacy of outward action.

Commentary on the Brahma Sūtras, I.1 (knowledge vs action) · Upadeśasāhasrī

The self (ātman) is never born and never dies: death touches only the body-mind compound, not the consciousness that witnesses it. In this respect Śaṅkara belongs to immortalism, the present life being a passage toward liberation. But the encoding calls for a nuance: what is immortal is not the individual soul, which unravels with its rebirths, but the impersonal self identical in all. For the one who has realized this identity, death is no longer even an event to fear, which adds a coloring close to "death is nothing": it can subtract nothing from what, in essence, was never in time.

It is never born and never dies; unborn, abiding, eternal, it is not slain when the body is slain.Bhagavad Gītā, II.20, as commented by Śaṅkara

Commentary on the Bhagavad Gītā, II.11-30 · Commentary on the Kaṭha Upaniṣad

Renunciation (saṃnyāsa) of the world and its enjoyments is held to be the condition most favorable to liberating knowledge: Śaṅkara's ideal sage is the wandering ascetic, detached from bodily pleasures as from every worldly good. Sensible pleasures bind one to the body-mind compound that the path invites one precisely to transcend. It is a pronounced asceticism, which does not hold the body to be evil in itself but subordinates it entirely to the quest for liberation.

Upadeśasāhasrī · Commentary on the Bhagavad Gītā

A single reality exists without a second, brahman, pure consciousness: neither matter nor the plurality of things are autonomous substances. The world of objects is not sheer nothingness, but it holds only an empirical reality (vyāvahārika), superimposed on brahman by ignorance, as one sees a snake where there is only a rope. This idealism is sui generis: it does not make the world the content of a finite mind, in Berkeley's manner, but refers it wholly to the one absolute consciousness of which things are merely the appearance. The question is thus not "mind or matter?" but "is the manifold real?", to which Śaṅkara answers by non-duality.

Brahman is real, the world is appearance, and the self is nothing other than brahman.verse attributed to Śaṅkara (Brahmajñānāvalīmālā)

Commentary on the Brahma Sūtras, I.1.1-4 · Upadeśasāhasrī

Śaṅkara distinguishes two levels of the absolute: a personal God, creator and object of devotion (saguṇa brahman, Īśvara), real at the empirical level, and, at the ultimate level, a brahman without attributes (nirguṇa), beyond all personhood and all relation. With respect to our axis this is not an atheism: the absolute is indeed the ground of everything. But neither is it the theism of a sovereign creator distinct from the world, for the personal God is himself a conventional figure that ultimate knowledge passes through. The tension between devotion and liberating knowledge is thus dissolved by the hierarchy of the two truths: God for the one who journeys, the impersonal for the one who knows, hence a theist but restrained position.

Commentary on the Brahma Sūtras, I.1 · Commentary on the Bhagavad Gītā