The four great Mahāvākyas, found in each one of our Vedas, are as follows:
Lakṣaṇa Vākya - Prajñānam Brahma - Consciousness is Brahman. (Aitareya-upaniṣad. (3-1-3) – Ṛg-veda).
Upadeśa Vākya - Tat Tvam Asi - That Thou Art. (Chāndogya-upaniṣad. (6.9.4 and 6.10.3) – Sāma-veda).
Anubhava Vākya - Ayam Ᾱtmā Brahma - This Self is Brahman (Māṇḍūkya-upaniṣad. (Mantra-2) – Atharva-veda).
Anusandhāna Vākya - Aham Brahma Asmi - I am Brahman (Bṛhadāraṇyaka-upaniṣad. (1-4-10) – Yajur-veda).
If one closely observes these four great formulae, we can see, in and through them, the entire system of the ‘Path of Meditation’ which Vedānta preaches.
As soon as the student reaches the Master’s feet, he has to define what is Brahman, and this statement of definition of the Eternal Truth – Lakṣaṇa Vākya – is found in the Ṛg-veda where it says, ‘Consciousness is Brahman’.
After this objective definition of Brahman, the Master turns to the disciples and directly points to their bosom and roars the truth, ‘That thou Art’, meaning that the Consciousness which is Brahman is not yonder there, somewhere behind the clouds, to be achieved at some time as a posthumous reward, but it is right here and now to be experienced as this Ᾱtman. This advice of the Master to the disciples – Upadeśa Vākya – is found in the Sāma-veda.
Now, the student, having heard the definition of Brahman, having understood from the Master’s own tongue that the essence in him is nothing other than Brahman Itself, retires to the solitude to meditate upon this objective Truth constantly and single pointedly until he discovers for himself, ‘This Self is Brahman’. This Mahāvākya or major text, since it is the road to Realisation for the seeker, is called the Anubhava Vākya. This is found in Atharva-veda.
Having decided for himself, from his own personal experience, gained during solitary and tranquil meditation, that this Ᾱtman is the Brahman, the student comes back to the Master. To the looks of enquiry of the Master, the boy answers with the cheer that beams out from his limpid eyes of peace and serenity that he had intimately experienced the Truth, and afterwards he is living constantly aware of the Truth that ‘Brahman Am I’. This major text, is, therefore, called Anusandhāna Vākya.
~Chinmayananda
from Vakya Vritti
‘Consciousness is Brahman’ (Prajñānam Brahma) is the definition of the ultimate Reality behind the ever-changing phenomenal world of things and beings. ‘That Thou Art’ (Tat Tvam Asi) is the teacher’s advice. The student in his seat of meditation realises subjectively in himself that ‘This Self within is Brahman’ (Ayam Ᾱtmā Brahma). Last comes the hallelujah that sings in the bosom of the now Liberated sage in the student and he in his sense of fulfilment and bliss immeasurable, confirms in a mad roar of joy and wonder, ‘I am Brahman’ (Ahaṁ Brahmāsmi).
~Chinmayananda