There are four Mahavakyas, or great statements in the Upanishads, which have a profound significance as pointers to Reality. They are: (1) Prajnanam Brahma - Consciousness is Brahman; (2) Aham Brahmasmi - I am Brahman: (3) Tat Tvam Asi - That Thou Art; (4) Ayam Atma Brahma - This Self is Brahman.

These Mahavakyas convey the essential teaching of the Upanishads, namely, Reality is one, and the individual is essentially identical with it. In the sentence, ‘Prajnanam Brahma’ or Consciousness is Brahman, a definition of Reality is given. The best definition of Brahman would be to give expression to its supra-essential essence, and not to describe it with reference to accidental attributes, such as creatorship etc. That which is ultimately responsible for all our sensory activities, as seeing, hearing, etc., is Consciousness. Though Consciousness does not directly see or hear, it is impossible to have these sensory operations without it. Hence it should be considered as the final meaning of our mental and physical activities. Brahman is that which is Absolute, fills all space, is complete in itself, to which there is no second, and which is continuously present in everything, from the creator down to the lowest of matter. It, being everywhere, is also in each and every individual. This is the meaning of Prajnanam Brahma occurring in the Aitareya Upanishad.

In the sentence, ‘Aham Brahmasmi,’ or I am Brahman, the ‘I’ is that which is the One Witnessing Consciousness, standing apart form even the intellect, different from the ego-principle, and shining through every act of thinking, feeling, etc. This Witness-Consciousness, being the same in all, is universal, and cannot be distinguished from Brahman, which is the Absolute. Hence the essential ‘I’ which is full, super-rational and resplendent, should be the same as Brahman. This is not the identification of the limited individual ‘I’ with Brahman, but it is the Universal Substratum of individuality that is asserted to be what it is. The copula ‘am’ does not signify any empirical relation between two entities, but affirms the non-duality of essence. This dictum is from the Brihadaranyaka Upanishad.

In the Chhandogya Upanishad occurs the Mahavakya, ‘Tat Tvam Asi’ or ‘That thou art’. Sage Uddalaka mentions this nine times, while instructing his disciple Svetaketu in the nature of Reality. That which is one alone without a second, without name and form, and which existed before creation, as well as after creation, as pure Existence alone, is what is referred to as Tat or That, in this sentence. The term Tvam stands for that which is in the innermost recesses of the student or the aspirant, but which is transcendent to the intellect, mind, senses, etc., and is the real ‘I’ of the student addressed in the teaching. The union of Tat and Tvam is by the term Asi or are. That Reality is remote is a misconception, which is removed by the instruction that it is within one’s own self. The erroneous notion that the Self is limited is dispelled by the instruction that it is the same as Reality.

The Mahavakya, ‘Ayam Atma Brahma’ or ‘This Self is Brahman’, occurs in the Mandukya Upanishad. ‘Ayam’ means ‘this’, and here ‘thisness’ refers to the self-luminous and non-mediate nature of the Self, which is internal to everything, from the Ahamkara or ego down to the physical body. This Self is Brahman, which is the substance out of which all things are really made. That which is everywhere, is also within us, and what is within us is everywhere. This is called ‘Brahman’, because it is plenum, fills all space, expands into all existence, and is vast beyond all measure of perception or knowledge. On account of self-luminosity, non-relativity and universality, Atman and Brahman are the same. This identification of the Self with Absolute is not any act of bringing together two differing natures, but is an affirmation that absoluteness or universality includes everything, and there is nothing outside it.