This great teaching occurs in the Chhandogya Upanishad. It is known as the upadesha vakya, or the statement of instruction.
The verse states “Now that which is the subtle essence – in it, all that exists has its self. That is the Truth. That is the Self. That thou art, Svetaketu. Tat Tvam Asi is the first of the Great Teachings that is delivered to the qualified disciple. In the statement, Tat (that) refers to the Supreme Self, Tvam (you) refers to the individual Self, and Asi (are) clearly shows the identical nature of the two. In other words, the statement emphasizes the oneness of the individual consciousness and the universal consciousness.
In this Upanishad, this same statement is repeated by the guru nine times to the disciple. For the aspirant who is just beginning the contemplation of the Great Teachings, no amount of emphasis can be too much. Continuously identifying ourselves with the body-mind and its limitations, we find it difficult to even comprehend this Truth. The seeker cannot see how he, the limited, ignorant individual, can be identical with the universal consciousness. To break this false identification, the guru repeatedly advises the disciple to go beyond the differences of name and form, and see the unity of Self. Since the individual Self and the universal Self are perceived by the disciple as two distinct entities, the guru has to start his instruction from this point. For the sake of the disciple, he refers to two distinct selves (thou and That), although in truth, only the one Self exists. Working with the assumption of duality, the disciple meditates and discovers that in truth, no two selves exist. That and thou both stand revealed as the non-dual, all-pervading Self alone.
