(Sanskrit: “Nondualism,” or “Monism”), most influential of the schools of Vedānta, an orthodox philosophy of India. While its followers find its main tenets already fully expressed in the Upaniṣads and systematized by the Vedānta-sūtras, it has its historical beginning with the 7th-century thinker Gauḍapāda, author of the Māṇḍūkya-kārikā, a commentary in verse form on the late Māṇḍūkya Upaniṣad.

Gauḍapāda builds further on the Mahāyāna Buddhist philosophy of Śūnyavā-da (“Emptiness”). He argues that there is no duality; the mind, awake or dreaming, moves through maya (“illusion”); and only nonduality (advaita) is the final truth. This truth is concealed by the ignorance of illusion. There is no becoming, either of a thing by itself or of a thing out of some other thing. There is ultimately no individual self or soul (jiva), only the atman (all-soul), in which individuals may be temporarily delineated just as the space in a jar delineates a part of main space: when the jar is broken, the individual space becomes once more part of the main space.

The medieval Indian philosopher Śaṅkara, or Śaṅkarācārya (Master Śaṅkara, c. 700–750), builds further on Gauḍapāda's foundation, principally in his commentary on the Vedānta-sūtras, the Śārī-raka-mīmāṃsā-bhāṣya (“Commentary on the Study of the Self ”). Śaṅkara in his philosophy does not start from the empirical world with logical analysis but, rather, directly from the absolute (Brahman (brahma)). If interpreted correctly, he argues, the Upaniṣads teach the nature of Brahman. In making this argument, he develops a complete epistemology to account for the human error in taking the phenomenal world for real. Fundamental for Śaṅkara is the tenet that the Brahman is real and the world is unreal. Any change, duality, or plurality is an illusion. The self is nothing but Brahman. Insight into this identity results in spiritual release. Brahman is outside time, space, and causality, which are simply forms of empirical experience. No distinction in Brahman or from Brahman is possible.

Śaṅkara points to scriptural texts, either stating identity (“Thou art that”) or denying difference (“There is no duality here”), as declaring the true meaning of a Brahman without qualities ( nirguṇa). Other texts that ascribe qualities (saguṇa) to Brahman refer not to the true nature of Brahman but to its personality as God (Īśvara).

Human perception of the unitary and infinite Brahman as the plural and infinite is due to human beings' innate habit of superimposition (adhyāsa), by which a thou is ascribed to the I (I am tired; I am happy; I am perceiving). The habit stems from human ignorance (ajñāna, avidyā), which can be avoided only by the realization of the identity of Brahman. Nevertheless, the empirical world is not totally unreal, for it is a misapprehension of the real Brahman. A rope is mistaken for a snake; there is only a rope and no snake, but, as long as it is thought of as a snake, it is one.

Śaṅkara had many followers who continued and elaborated his work, notably the 9th-century philosopher Vācaspati Miśra. The Advaita literature is extremely extensive, and its influence is still felt in modern Hindu thought.