What are the mahAvAkya-s ?
mahAvAkya-s or great-sentences succinctly state the pAramArthika tattva, namely the unity of Atman and brahman. Their greatness arises from the
fact that they are capable of destroying avidyA by themselves (with no other aid) in the qualified persons. Thus in the pai.ngala upanishhad 3.1 we
have: sa-hovAcha-yAGYavalkyaH-tat-tvaM asi, tvaM tad-asi, ... anusandhAnaM kuryAt.h - that yAGYavalka said, "That thou art, Thou that art, ...
should be deeply reflected upon".
The pa.ncIkaraNa (19) says the mahAvAkya-s are four in number as does the shuka-rahasya-upanishhad (12). This upanishhad declares: atha-mahAvAkyAni chatvAri, yathA praGYAnaM brahma, aham brahmAsmi, tattvamasi, ayamAtma brahma - The mahAvAykyas are four, they are 'brahman is consciousness', 'I am brahman', 'That thou art' and 'This Atma is brahman'. In the order stated above, they are to be found in aitareya 5.3, bR^ihadAraNayaka 1.4.10, chhAndogya 6.8.7, and mANDUkya 1.2. These upanishhad-s belong to the R^ig, shukla-yajus, sAma and athavaNa veda-s respectively. Thus, the number of mahAvAkya-s is the same as the number of veda-s. It is interesting to note that except praGYAnam brahma, the rest equate the Atman and brahman directly.
The four mahAvAkya-s and their variants are to be found in other upanishhads also. For example, the aghamarshhaNa sUktam in the mahAnArAyaNa upanishhad (kR^ishhNa yajur-veda) states aham-asmi-brahma-aham-asmi. The kaivalya upanishhad (kR^ishhNa yajur veda) states tat-tvam-asi-tvam-eva-tat.h. sha.nkarAchArya and his followers usually take tat-tvam-asi as the paradigmatic mahAvAkya. But, it should not be forgotten that the same kind of exegesis can be applied to the other mahAvAkya-s also.