Nididhyāsana

Nididhyasana is profound and repeated meditation. In Advaita Vedanta and Jnana Yoga, it is meditation on the mahavakyas, great Upanishadic statements such as "That art Thou", to realize the identity of Atman and Brahman. It is the fourth step in the training of a sisya, consisting of preparatory practices, listening to the teachings as contained in the sruti, reflection on the teachings, and nididhyasana.

Four stages of practice

In Advaita Vedanta, Nididhyasana is the final step in the correct understanding of the meaning of the Mahavakyas. Classical Advaita Vedanta emphasizes the path of Jnana Yoga, a progression of study and training to attain moksha. It consists of four stages:Samanyasa or Sampattis, the "fourfold discipline", cultivating the following four qualities:

* — The ability to correctly discriminate between the eternal substance and the substance that is transitory existence.
* — The renunciation of enjoyments of objects in this world and the other worlds like heaven etc.
* — the sixfold qualities,
** Śama.
** Dama.
** Uparati.
** Titikṣa.
** Śraddhā.
**Samādhāna.
* — The firm conviction that the nature of the world is misery and the intense longing for moksha.Sravana, listening to the teachings of the sages on the Upanishads and Advaita Vedanta, and studying the Vedantic texts, such as the Brahma Sutras. In this stage the student learns about the reality of Brahman and the identity of atman;Manana (reflection), the stage of reflection on the teachings;Nididhyāsana, the stage of meditation on the truth "that art Thou".

Nididhyasana is a rational and cognitive process, which differs from dhyana. It is necessary for gaining Brahmajnana:
Nididhyasana done independently of sravana does not lead to the realization of the Atman.

Explanation

Advaita Vedanta

Brihadaranyaka Upanishad defines Nididhyasana as the meditation for the sake of direct vision. Yajnavalkya tells his wife –
Adi Shankara took a subitist stance, stating that hearing the mahavakyas directly leads to insight, without the need for nididhyasana.
According to Suresvara, Nididhyasana is the culmination of the practice of sravana and manana, which is an indirect intuition of Brahman and does not mean meditation but knowledge i.e. understanding the meaning of the Sruti on the basis of vacya-vacaka relation underlying the mahavakya. Suresvara states:-
Nididhyasana consists in acquisition of vakyarthajnana and this verse explains the purport of sunisnata.
According to Vacaspati, sravana, manana and nidihyasana are a chain of causes contributory to the knowledge of the oneness of Brahman. The Vivarna school considers sravana as the principal cause but Suresvara treats sravana and manana to be co-existent, these two culminate into nididhyasana.
The late mediaeval Advaita Vedanta tradition added yogic samadhi as a means to discern the true Self or Atman. The Vivekacudamani, incorrectly attributed to Shankars, states:

Dvaita Vedanta

According to Madhva the knowledge acquired by study and stabilized by reflection is made the basis of steady contemplation ; these are the three stages of inquiry that take the form of Dhyana. Radhakrishnan has defined Nididhyasana as "the process by which intellectual conscience is transformed into a vital one there is stillness, a calm in which the soul lays itself open to the Divine".

Neo-Vedanta

According to Michael James, who gives an Advaita Vedanta interpretation of Ramana Maharshi, Ramana's self-enquiry is the same as Nididhyasana and atma-vichara.