All this that we see without is Brahman. This Self that is within is Brahman.
This Self, which is one with OM, has three aspects, and beyond these three, different from them and indefinable - the Fourth.
The Fourth, the Self, is OM, the indivisible syllable. This syllable is unutterable, and beyond mind. In it, the manifold universe disappears. It is the supreme good - One without a second. Whosoever knows OM, the Self, becomes the Self.
from Mandukya Upanishad