The Upanishads

The indefinite term "That" is used in the Upanishads to designate the Invisible-Absolute, because no word or name can fully define It. A finite object, like a table or a tree, can be defined; but God, who is infinite and unbounded, cannot be expressed by finite language. Therefore the Rishis or Divine Seers, desirous not to limit the Unlimited, chose the indefinite term "That" to designate the Absolute.

In the light of true wisdom the phenomenal and the Absolute are inseparable. All existence is in the Absolute; and whatever exists, must exist in It; hence all manifestation is merely a modification of the One Supreme Whole, and neither increases nor diminishes It. The Whole therefore remains unaltered.