On what basis are 4 mahavakyas chosen?

Question

There are many upanishadic statements.

I want to know why these four only are considered mahavakyas?

1. Prajnanam Brahma (प्रज्ञानम् ब्रह्म)
2. Aham Brahma Asmi (अहम् ब्रह्म अस्मि)
3. Tat Tvam Asi (तत् त्वम् असि)
4. Ayam Atma Brahma (अयम् आत्मा ब्रह्म)

Which other schools consider them as mahavakyas?

Are there any Upanishad statements which indicate duality and thus they can be made mahavakyas? I know one. For eg: brahmaiva san, brahmapyeti "Becoming brahman, one enters brahman".(Brh. Ar: 4.4.6). This indicates two even after liberation, since author of action must be different from object.

A non advaitin can choose this as a mahavakya and interpret advaitic mahavakyas differently.

Why not this be a mahavakya?

The full verse translation from here is:

And here there is this verse: "To whatever object a man's own mind is attacbed, to that he goes strenuously together with his deed; and having obtained the end (the last results) of whatever deed he does here on earth, he returns again from that world (which is the temporary reward of his deed) to this world of action." 'So much for the man who desires. But as to the man who does not desire, who, not desiring, freed from desires, is satisfied in his desires, or desires the Self only, his vital spirits do not depart elsewhere,- being Brahman, he goes to Brahman

Paingal Upanishad seems to have those statements (but only two of them match), since Adi Shankara cited this upanisahd according to this website, did he himself quote those verses?

In any case, I am more looking towards logical explanation of why these four are chosen? Who is the first advaitin to say these four as mahavakyas, which means when did they enter into advaitin books for the first time and how did he justify choosing these four over other statements?

Answer 1

महावाक्यानि (Mahvavakyas) are the great sayings and taken as a conclusion of Vedas. These are cited many times to establish the message of Vedas in concise manner.

Non-duality is a well established fact in Sanatan Dharma. Although you may find other verses which defy non-duality like the one you mentioned, they are merely there to signify some point (other than duality of Brahman and Jiva). Without using duality conveying a message is impossible. Such verses cannot be taken literally and hence cannot be quoted independently. Hence they aren't mahavakyas because they aren't the conclusion.

Objection: It could be the case that verses that convey non-duality are there to signify some other point (and hence cannot be taken literally) and the ones which convey duality are the actual ones which should be taken literally.

Me: No, this cannot be the case. Because the verses which convey non-duality, their sole purpose is to convey that truth. While the one that defy non-duality do so indirectly for the purpose of illustration of some point for the lack of better language. Further, you cannot find a single verse that directly defies non-duality and conveys duality while I can provide you verses that directly defies duality and establishes non-duality.

He goes from death to death who sees multiplicity in It (Kath. Up. 2.1.11)[http://upanishads.org.in/upanishads/3/2/1/11]

Adi Shankra also says in Mandukya Karika 3.13:

The Śāstras as well as the sages like Vyāsa, etc., extol the identity of Jīva and the Supreme Self through the negation of all differences—the conclusion arrived at by reasoning and supported by the scriptures. Further, the experiences of multiplicity which are natural (to the ignorant) and common to all beings—the view propounded by those who do not understand the real import of the Śāstras and who indulge in futile reasoning—have been condemned thus: “But there is certainly nothing corresponding to the dual existence,” “Fear arises from the consciousness of duality,” “If he sees the slightest difference (in Ātman) then he is overcome with fear,” “All this is verily Ātman, “He goes from death to death who sees here (in this Ātman) multiplicity.” Other Knowers of Brahman as well as the scriptures (quoted above) extol identity (of Jīva and Brahman) and condemn multiplicity. Thus alone this praise and condemnation can easily be comprehended; in other words, it accords with reason. But the false views (vainly) advanced by the logicians, not easy of comprehension, cannot be accepted as facts (Truth).

PS: Also I would request everyone who disagrees with these Mahavakyas to please go and debate with Advaitins. They are always open to debate if you go with an open mind. If sages of the past hadn't debated saying the opposite side is of another sect and hence not fit to debate, we wouldn't have such a rich and vibrant culture. There were no Advaita and non-Advaita in earlier times, lets not create such differences. We always have been seekers of truth from time immemorial.

Answer 2

In A Concise Encyclopaedia of Hinduism (Vol. 2), Swami Harshananda explains why only those 4 lines are chosen as the mahāvākyas:

mahāvākyas ('great sentences')

The post-Śaṅkara writers on Advaita Vedānta have mentioned four sentences, taken from four Upaniṣads and belonging to the four Vedas, which teach the unity of the ātman and Brahman, as four 'mahāvākyas' or great sentences. They are: prajñānam brahma ('Consciousness is Brahman') from the Aitareya Upaniṣad (5.3) of the Ṛgveda, aham brahmāsmi ('I am Brahman') from the Bṛhadāraṇyaka Upaniṣad of the Yajurveda (1.4.10); tat tvam asi ('You are That') from the Chāndogya Upaniṣad (6.8.7) of the Sāmaveda and ayamātmā brahma ('This ātman is Brahman') from the Māṇḍūkya Upaniṣad (2) of the Atharvaveda.

So it has to do with both the core teaching of Advaita Vedānta and that each of the lines is taken from a different Veda.

BTW, according to this bio, Upaniṣadbrahmendra, an Advaita teacher, has come up with a list of 1008 mahāvākyas:

A second major work of Upaniṣadbrahmendra in the Upaniṣadprasthāna is the collection of one thousand and eight Mahā-vākyas from all the Upaniṣads — the Aṣṭottarasahasramahāvākyāvali — and expositions of these in a series of commentaries, the Prabhā, the Lochana, the Vivaraṇa, and the Kiraṇāvali. Even at the beginning of his commentary on the Upaniṣads, he has shown that the Mahavākyas are not just four, but many more.