The Upanishads developed two fundamental concepts. The first one is "the self" and its connection to the "Brahman". The later term, was often treated as not just the power of the mantras or of the sacrifice but as a cosmic principle. Yet, the Upanishads added something up to the mentioned definition. Brahman, according to The Upanishads, is not only a cosmic principle; it is also the self of man. For the Upanishads to know Brahman is to know all, since Brahman is "this ALL". Basically, a person who knows the Brahman and his atman "has no need to fear death, because atman is complete in itself, self-existent, wise and immortal." A problem arises when Hindu people attempt to understand the Upanishads' approaches or definition of Brahman. As it was mentioned before, The Upanishads added a new concept to the Brahman and this led to different speculation within Hinduism. The Brihadaranyaka and Chandogya, the two oldest Upanishads, defined and explained the reason of seeing the Brahman and the self as one unit. According to them Brahman is the totality of the existent and the nonexistent. For them, the only way to identify Brahman was the atman or self of man. The Brihadaranyaka interpretations of the self is "That self is not this, not that. It is incomprehensible for it is not comprehended. It is indestructible for it is never destroyed. It is unattached for it does not attach itself. It is unfettered. It does not suffer. It is not injured."
On the other hand, Chandogya see the self as "The self is when a man is asleep, composed, serene, knows no dream. But in that state there is no awareness even of personal existence." Also for it, "the body is only the support of the deathless, bodiless self. Freed from the body, the self rises up and reaches the highest life where he appears in his own form as the supreme person." Basically, for both points of view, the self is essentially free of the body.
As it can be appreciated the Upanishads has several ways to describe self. As stated by Yajnavalkya, the self is the person and this has two states, one is the state of this world and the other is the state of being in the other world. Between these states there is a twilight state. The self in this "twilight state" is the creator and as a creator the person in dreaming sleep "takes along the material of this all-embracing world with all its contents, himself tears it apart, himself builds it up."