I heard that तत्त्वमसि (Tat Tvam Asi) is the Mahavakya. So, I want to know what are the Mahavakyas and which are they?
The Mahavakyas are "Grand Pronouncements" or "Great Sayings" usually taken from Upanishads. They have profound significance in Hinduism.
The four Mahavakyas (महावाक्यानि) are as follows:
1. Prajñānaṃ Brahma (प्रज्ञानं ब्रह्म)
- From Aitareya Upanishad 3.3 of the Rig Veda.
- Also called लक्षणा वाक्य. It is about (pure) consciousness of Brahman
2. Tat Tvam Asi (तत्त्वमसि)
- From Chandogya Upanishad 6.8.7 of the Sama Veda
- Also called उपदेश वाक्य. It is about to preach that "You are that (Brahman)".
3. Ahaṃ Brahmāsmi (अहं ब्रह्मास्मि)
- From Brhadaranyaka Upanishad 1.4.10 of the Yajur Veda
- Also called अनुभव वाक्य. It is about to realize that I am Brahman.
4. Ayaṃ Ātmā Brahma (अयं आत्माब्रह्म)
- From Mandukya Upanishad 1.2 of the Atharva Veda
- Also called अनुसंधान वाक्य. It is about to research or make understand that The self (Atman) is Brahman.
Though there are also other Mahavakyas besides these but these four are considered most important.
Paingala Upanishad in chapter 3 lists the Mahavakyas:
In this chapter, Paingala asked Yajnavalkya to offer an exposition on the Mahavakyas(Great sayings of the Vedas). To which Yajnavalkya replied the following:
- Tattvamasi (That art thou),
Tvamtadasi (Thou art That),
Twambrahmasi (Thou art Brahman),
- Ahambrahmāsmi (I am Brahman).
Adi Shankaracharya mentioned the maha-vakya twice in his writings:
In Aitareya Upanishad bhasya 1.3.13
sa kadācitparamakāruṇikena ācāryeṇātmajñānaprabodhakṛcchabdikāyāṃ vedāntamahāvākyabheryāṃ tatkarṇamūle tāḍyamānāyām? etameva sṛṣṭyādikartṛtvena prakṛtaṃ puruṣaṃ puri śayānamātmānaṃ brahma bṛhat tatamaṃ takāreṇaikena luptena tatatamaṃ vyāptatamaṃ paripūrṇamākāśavat pratyabudhyata apaśyat। katham idaṃ brahma mama ātmanaḥ svarūpamadarśaṃ dṛṣṭavānasmi।
When, however, some preceptor possessing great compassion beat at the root of his ears the kettle-drum of the mahavakyas or key notes of the Vedanta, whose sound wakes up the knowledge of the A'tman, he saw his Self as the Brahman, the Creator dwelling in the body, yet all-pervading like the A'kas. The word tatamam, having another letter to dropped, should be tatatamam, meaning all-pervading. He cried " I have seen this Brahman, the real essence of my A'tman."
He leaves it quite open. He doesn't define them, they seem to be pedagogical devices.
Another in Sariraka bhasya 1.3.33:
na hi mahāvākye 'rthapratyāyake'vāntaravākyasya pṛthakpratyāyakatvamasti yathā na surāṃ pibet iti nañvati vākye padatrayasaṃbandhātsurāpānapratiṣedha evaiko'rtho'vagamyate na punaḥ surāṃ pibediti padadvayasaṃbandhātsurāpānavidhirapīti।
For in general any minor syntactical unity, which is included in a more comprehensive syntactical unity conveying a certain meaning, does not possess the power of expressing a separate meaning of its own. Thus, for instance, we derive, from the combination of the three words constituting the negative sentence, '(Do) not drink wine,' one meaning only, i.e. a prohibition of drinking wine, and do not derive an additional meaning, viz. an order to drink wine, from the combination of the last two words, 'drink wine.'
Here the mahavakya is '(Do) not drink wine'