Mahavakyas are the four ‘great sayings’ of the Upanishads.
We should not expect to find an original red-lined version of the Upanishads with these four verses highlighted and raised to a special philosophical status. For one thing, we know that the early tradition was an oral tradition, handed down from teacher to student. Furthermore, it is not known with much certainty why or when these four verses, out of the many thousands of verses of the Upanishads, were designated mahavakyas.
The verses in question, at least according to the Advaitic tradition, are: aham brahmasmi, tat tvam asi, ayam atma brahma, and prajnanam brahma. What we do know is that they were chosen by Acharya Adi Shankara to represent mantras attached to the four mathas, spiritual centres, in the four corners of India, each of the four taken from one of the four Vedas.
Thus, we find that:
- Aham brahmasmi; I am Brahman of the Brihadaranyaka Upanishad, Yajur Veda, is connected with the Shringeri Sharada Pitha.
- Tat tvam asi; Thou art That of the Chandogya Upanishad, Sama Veda, with the Dvaraka Pitha.
- Ayam atma brahma; This Self is Brahman of Mandukya Upanishad, Atharva Veda, with the Jyotirmath Pitha.
- Prajnanam brahma; Brahman is pure consciousness of Aitareya Upanishad, Rig Veda, with the Govardhana Pitha.
It is probably due to the tremendous influence of Acharya Shankara that these four mantras are now considered by many to be the authentic mahavakyas, but when did the designation become more or less ‘official’ does not seem to be known.
Source – Excerpt from the article ‘The Significance of the Mahavakyas by Swami Atmajnanananda in Prabuddha Bharata January 2020 page 58.