Upanishad says: ~ sarvam khalvidam brahma – all this (universe) is verily Brahman. By following back all of the relative appearances in the world, we eventually return to that from which it is all manifest – the non-dual reality (Chandogya Upanishad).
What this means is by examining the waking, dreaming states, we return to that from which all manifests – non dual reality, dreamless sleep state.
Jesus said examine yourself (gospel of thomas). This is what he means, after all what else is there?