Wherefrom do all these worlds come? They come from space [monad]. All beings arise from space, and into space they return: space is indeed their beginning, and space is their final end.
The Chandogya Upanishad is one of the "primary" (mukhya) Upanishads. Together with the Jaiminiya Upanishad Brahmana and the Brihadaranyaka Upanishad it ranks among the oldest Upanishads, still dating to the Brahmana period of Vedic Sanskrit (before sixth century BCE). It is associated with the Kauthuma Shakha of the Samaveda. It figures as number 9 in the Muktika canon of 108 Upanishads.