The remainder of the Māṇḍūkya Upaniṣad reiterates what has already been covered on the nature of Aum and Ātman as a mystical science and sacred formula for the ages. The translation is by Swami Krishnananda and commentary will be offered for the final segment on the Divine Turīya, since it comes closest to the Nature of the Unborn.
This identical Ātman, or Self, in the realm of sound is the syllable OM, the above described four quarters of the Self being identical with the components of the syllable, and the components of the syllable being identical with the four quarters of the Self. The components of the Syllable are A, U, M.
Vaiśvānara, whose field is the waking state, is the first sound, A, because this encompasses all, and because it is the first. He who knows thus, encompasses all desirable objects; he becomes the first.
Taijasa, whose field is the dream state, is the second sound, U, because this is an excellence, and contains the qualities of the other two. He who knows thus, exalts the flow of knowledge and becomes equalised; in his family there will be born no one ignorant of Brahman.
Prājña, whose field is deep sleep, is the third sound, M, because this is the measure, and that into which all enters. He who knows thus, measures all and becomes all.
The fourth is soundless: unutterable, a quieting down of all relative manifestations, blissful, peaceful, non-dual. Thus, OM is the Ātman, verily. He who knows thus, merges his self in the Self – yea, he who knows thus.
Aum śantih; śantih; śantih
Aum Peace! Peace! Peace!
Śaṅkara: The (soundless) is that which has no parts (sounds, etc., or letters). This partless Aum which is the fourth, is nothing but Pure Ātman. It is incomprehensible, because both speech and mind which correspond to the name and the object disappear or cease; the name and the object (that is indicated by the name) which are only forms of speech and mind cease or disappear (in the partless Aum). It is the cessation of the (illusion of) phenomena and all bliss and is identical with non-duality. Aum, as thus understood, has three sounds which are the same as the three quarters and therefore Aum is identical with Ātman. He who knows this merges his self in the Self which is the Highest Reality. Those who know Brahman, i.e., those who realise the Highest Reality merge into Self, because in their case the notion of the cause which corresponds to the third quarter (of Ātman) is destroyed (burnt). They are not born again, because Turīya is not a cause.
* Soundless—It is because Amātra Aum cannot be expressed by any sound. It is relationless and therefore it cannot be described as the substratum of three other sounds. Sound points out, by contrast the soundless Aum. All sounds must, at some time or other, merge in soundlessness. This Amātra Aum is identical with Turīya Ātman.
(Here ends the Māṇḍūkya Upaniṣad with the Commentary of Sankara.)
Unborn Commentarius: The Māṇḍūkya Upaniṣad is a sacred science of Aum and Its attributes. The Upaniṣads themselves are an unraveling of Aum with the former leading the pact. Here, we can see that Turīya consists in all that is not of our Actual-True Self and thus dissolves away as if in warm liquid. Turīya is the zero-point of the Via Negativa wherein one is no longer attached to any phenomenal forms and their interrelationships. There is a feeling that one has now completely done away with the affairs of the world. This is True and irrevocable Liberation from the samsaric cycle of birth, death, and rebirth—Moksha. There is now no more space and time and a return to the deathless and imageless cradle of the Unborn Mind and Spirit.