The term Upanishad means "those who sit near"
and implies listening closely to the secret doctrines of a spiritual
teacher. The Upanishads are a collection of Indian speculations on the
nature of reality and the soul and the relations between these two. The
texts date from around 600 BC. They often give the impression of a
building under construction - an ongoing exploration of themes not yet
fully resolved. Like the Bhagavad Gita, Upanishads are the work of
several hands.The theme that unites these writings, is the search for an underlying unity linking everything we see and think. That unity is called Brahman. In most texts, Brahman is said to be identical with Atman, the world soul, which is also identical with the Atman of each individual. In this sense every individual is united with the cosmos, and only needs to realize this fact to reach fulfillment.
The Upanishads are, thus, essentially pantheistic. However, their quest for unity went at times too far. Since the world we see is highly diverse, some texts deduce that if the cosmos is unitary, then appearances must be Maya (a magical trick). The self, the presence of the cosmic Self within each person, is the 'Real' world - a world of pure thought, which cannot be apprehended by the senses. Meanwhile the real sensible world of life and love, birth and death, is dismissed as no more than a dream projected by the self. Once a person realizes this, the illusion falls away and they become one with the all.
This view - similar in essence to most schools of Buddhism - contrasts strongly with realistic pantheisms which accept the material world as real. Idealistic pantheism fails to explain why we all dream the same dream - why the detail of the sensible world is shared by everyone present. It cannot explain why we should start to 'dream' the world at all, nor why there should be a `we' to dream. The belief that reality is illusion is perhaps the greatest illusion of all. It leads to indifference, inaction, insensitivity, world-blindness, death in life. Indeed in the Upanishads all these are praised as the highest wisdom.
No life can be led in an illusory world - even hermits and monks have to eat, excrete, and sleep. Thus, they are forced to acknowledge, in action, the reality of the world they deny in theory, and their own lives disprove their beliefs. The only life that is consistent with full acceptance of idealist pantheism is a hunger strike until death. The sage who wishes to survive another day must forget his wisdom.