MANDUKYA Upanishad is attributed to the Atharvaveda. It is extremely short, composed in prose, stated in terms, the meanings of which are not easy to understand.
It begins by a positive statement regarding the Universe, which is described as the exposition of the mystic syllable, AUM. It says:
"AUM is this imperishable word, AUM is the Universe and this is the exposition of AUM."
It is then declared that AUM is the past, the present and the future. And, finally, it is declared that all else that may exist beyond the bounds of Time, that too is AUM.
A further positive statement regarding the Universe is made next:
"All this Universe is the Eternal Brahman, this Self is the Eternal, and the Self is four-fold."