Whenever a totally transcendent and impersonal Absolute such as Brahman is accepted the question has to be raised that if the Absolute is beyond anything that humankind can comprehend then how can it be known in any way? The answer to this question can only be a negative one unless there is some part of manifest existence which is linked in some day - even though indescribably so - with the totally transcendent Absolute. It is this link which is so important in the mystical aspects of religion like Vedanta. The Upanisads answer the question by identifying the innermost essence of each manifest entity, the atman, with the Unmanifest Source, Brahman: It is this principle which is the key to Upanisadis thought and which forms the core of Hindu belief. There are times when this relationship between Brahman and atman will be dual - the atman only being a part of Brahman and not wholly identifiable with It. There are certainly such dualistic tendencies in the Upanisads themselves but the main message of the Upanisads is a monistic one Brahman and atman is one and the goal of existence was (and still is) the experience of such oneness, the self becoming the Self. The most well-known words which express this principle come from the Chandogya Upanisad and they are Tat tvam asi `That art thou'.

     The story behind these words is that of Svetaketu, a young boy who at the age of twelve embarked on his period of study with a guru, the first asrama. When he returned twelve years later at the age of twenty-four, somewhat conceited and arrogant with the amount of knowledge he had acquired, his father, Uddalaka, asked him whether he had requested from his guru that instruction `by which the unhearable becomes heard, the unperceivable becomes perceived, the unknowable becomes known?' Svetaketu's inability to see how such a question could be answered prompts his father to explain that just as there is one substance, clay, from which all clay vessels come, or gold, from which all gold objects come, so there is one Source, that of Being, from which the world of not-being is derived. But the examples of clay are easy to be seen and therefore Svetaketu's father directs his son to more subtle ideas. He asks his son to say what he can see when he cuts the fruit from a banyan tree in half. When it reveals the tiny seeds, his father asks him to cut one of the seeds in two and again say what he can see. This time Svetaketu has to admit that he can see nothing yet he knows that something within the seed must cause it to grow to the mighty banyan tree. Uddalaka tells his son:

     My dear, that subtle essence which you do not perceive, verily; my dear, from that very essence this great nyagrodha tree exists. Believe me, my dear. That which is the subtle essence, this whole world has for its self. That is true. That is the self that art thou Svetaketu.

     Uddalaka illustrates the same principle by asking his son to place some salt in water. The following morning he asks his son to bring him the salt from the water, but of course it has dissolved. And Uddalaka explains to his son that Pure Being cannot, likewise, be perceived although it is the essence of the cosmos; the Source of everything in it, and is identical to the deepest part of the self.

     The words Tat tvam asi equate That as Brahman with the true inner self, the ' sphere of space in the chasm of the heart ', which is the atman. The Upanisads, as later Hinduism, differentiate between the embodied self (jivatman) which is perishable, impermanent, and transient, and the real Self (atman) which is permanent: The atman, being present in the body seems to be connected with it, with its desires, aversions, emotions, joys, sadness and so on. Bahadur gives a very good analogy here of clear glass (the atman) placed on a red cloth (the personality and its involvement with the world). The glass seems red until it is separated from the cloth. Likewise the self must be freed from its egoistic involvement with the world in order to know itself as the true Self which is Brahman. But this true Self can only be experienced within: all the knowledge which Svetaketu had acquired had brought him no closer to it. Succinctly, then, the Upanisads teach that: behind all of the spatial swirl and temporal flux of the world as it is experienced by the senses is a subtle, pervasive, timeless, and unchanging reality that is identical to the undying essence of the human being as well. It is the spacial swirl and temporal flux of the world which prevent the individual from experiencing the atman. The Upanisads often state that the only time the pure Consciousness of atman is really experienced is in dreamless sleep, albeit in a transient way. The identity of the atman to Brahman is often depicted by the rather good analogy of space in a jar and space outside it. The space in the jar is temporarily confined, as the atman is confined by the bodily self But the space in the jar is still the same as the space outside it and when the jar breaks, the space is one, just as atman and Brahman are really one. Whatever exists must contain some of this space but is at no second of time separated from the greater space.