The central truth of the Upanishads, the essence of the Vedanta, the goal of all wisdom and all spiritual practices, the reality of all realities, the quintessence of truth is that the individual self, the embodied soul, the Jiva, is identical with the Supreme, the Absolute, the Brahman, and this is what is meant by the great utterance of Self-revelation of the Upanishads: 'Tat Tvam Asi', That thou art. The nature of the inner essence of yourself and the nature of Brahman is one and the same, free from all change, impersonal and all-pervasive, infinite consciousness. You are not separate from Brahman: you are no other than the Brahman Itself and It alone you are, the Existence-Knowledge-Bliss Absolute, and this indeed, is the final truth which the sages of Realisation declare through their lives and this is the culmination of all spiritual experiences of the soul. . . Tat Tvam Asi, That thou art.
Tat Tvam Asi is the Abheda-Bodha-Vakya of the Upanishad which is also the Upadesa-Vakya instructed by the Brahma-Srotri Brahma-Nishtha Guru to the Adhikari or the disciple. The non-separateness of Jiva and Brahman is brought out by this great aphoristic precept which Sage Uddalaka repeated nine times to his son and disciple Svetaketu.
'Thou art That' is the meaning carried out by this declaration of the Sruti. It has got a superficial verbal meaning or Vachyartha and an essential indicative meaning or Lakshyartha.
The individual and the cosmic, Avidya and Maya, Jiva and Isvara, Atman and Brahman are the aspects which correspond themselves to the meaning of the terms 'Tat' and 'Tvam', 'That' and 'Thou'. That identity of the two is brought out by the following illustration.
A person Devadatta is seen by me in January at Delhi. I recognise him and say 'This is Devadatta'. I go to Agra on another occasion and find the same Devadatta there in April and explain, 'This is that Devadatta', 'Soyam Devadattah', referring to the identity of the person seen at two places at two different times. The superimpositions which appear in the 'January-Delhi-Devadatta' and the 'April-Agra-Devadatta' are ignored and only the real 'Devadatta' is taken into account. The references to Time and Space. . . January, April, Delhi, Agra. . . are only temporary and relative, for the Devadatta who was in Delhi during the month of January cannot be different from the Devadatta who came to Agra during the month of April, because the person is the same, though the place and the time are different. Thus the identity of the two Devadattas is determined.
The individual and the cosmic persons respectively limited by Avidya and Maya, namely, the Jiva and the Isvara are two personalities differentiated by space and time. When the verbal meaning or the Vachyartha of the Mahavakya is taken, the Jiva is asserted to be Isvara himself in the Pindanda. The Visva, Taijasa and Prajna of the microcosm or Pindanda correspond closely to the Virat, Hiranyagarbha and Isvara of the macrocosm or the Brahmanda. Thus the Jiva is an exact copy of or is identical with Isvara.
But the Lakshyartha or the indicative meaning of the sentence is brought out by the illustration in the story by 'Soyam Devadattah' or 'This is that Devadatta'. The limitations are cast off and the essence only is taken. Atman limited by Avidya is Jiva; Brahman limited by Maya is Isvara. When the Avidya of Jiva is cast off and the Maya of Isvara is ignored, what remains is Atman instead of Jiva and Brahman instead of Isvara. Just as the Devadatta of Delhi was the same as the Devadatta of Agra, the Reality of Isvara and the Reality of the Jiva are one and the same. Hence Atman is identical with Brahman. 'Thou' stands for the Atman and 'That' for Brahman, and the word 'art' or 'Asi' signifies the identity of the two as the one Akhanda-Ekarasa-Satchidananda-Ghana.