According to vedanta, technically speaking, it is that pure consciousness which we talked about earlier. That consciousness gets the name hiranagarbe associated with maya and the cosmic mind. It gets the name virat or visarupe, what urjuna saw in the eleventh chapter of the pagawadgita. But did you notice we have landed up ourselves in a nice little soup? Big problem. Why when you say that thou art, you are that huge contradiction. I this individual being. How can i be that the creator, preserver and destroyer of the universe,. whih is associated with maa, which is associated with the cosmic mind, which is
Sage Uddalaka asks his son Svetaketu: What is that knowledge by which we hear the unhearable, perceive what cannot be perceived and know what cannot be known? Inspired from this story from the Chandogya Upanishad, Swami Sarvapriyananda teaches the great saying (mahavakya) - Tat Tvam Asi (That Thou Art).
This discourse is also inspired from a verse from the Brihadharanyaka Upanishad (Mantra 4.4.12) as taught in the book, Pancadasi by Vidyaranya: "If a man knows the Self as ‘I am this,’ then desiring what and for whose sake will he suffer in the wake of the body?"
Mantra 4.4.12:
आत्मानं चेद्विजानीयादयमस्मीति पूरुषः ।
किमिच्छन्कस्य कामाय शरीरमनुसंज्वरेत् ॥ १२