There are three layers to the universe. In the lower, Tai Ching, and the middle, Shan Ching, the hindrance of a physical bodily existence is required. Those who fail to live consistently in accord with the Tao reside here. In the upper, Yu Ching, there is only Tao: the bondage of form is broken, and the only thing existing is the exquisite energy dance of the immortal divine beings.

Hua Hu Ching: Teachings of Lao Tzu (ed. 1992)