Commentary by Shankaracharya (adapted):
(1) The first chapter, that is to say, the Mandukya Upanishad proper, declared the reality of Non-duality on scriptural evidence through the explanation of AUM.
(2) The second chapter established the same truth with the help of reason and scripture (scripture supporting what was proved by reason), by demonstrating the unreality of the distinctions implied by the external objects of experience.
(3) The third chapter, dealing with Non-duality, established it directly, on the authority of reason and scripture, and concluded with the statement: “The supreme truth is that nothing ever is born." The Knowledge of Non-duality, which alone is the real import of the Vedas, is challenged by the dualists and by the nihilists of the well-known Buddhist school. But the teachings of these schools contradict each other and their followers’ reasoning is vitiated by the blemishes of attachment to their own opinion and aversion for that of others. Hence the knowledge they claim is false knowledge. The philosophy of non-dualism is true philosophy because it is free from these blemishes. (Two of the tests of Reality are that it must be free from contradiction and conflict and must be conducive to the welfare of all. Ultimate Reality is that by the knowing of which everything can be known. The Knowledge of Non-duality satisfies these conditions.)
(4) The purpose of the fourth chapter, called Alatasanti, or the Quenching of the Fire-brand, is to bring to a conclusion the discussion of the philosophy of non-dualism. This is done by showing in detail that other systems, on account of the mutual contradictions inherent in them, cannot be called true philosophy. The method followed is called the method of disagreement, or avitanyaya. (This is one of the processes of inference, the other being known as the method of agreement. The second chapter, following the method of agreement, has shown that what is caused or brought into existence is unreal. In the present chapter it will be shown that what is not unreal is also not caused.)
AUM is All
Harih Aum! AUM, the word, is all this, the whole universe. A clear explanation of it is as follows: All that is past, present and future is, indeed, AUM. And whatever else there is, beyond the threefold division of time—that also is truly AUM. (1.1)
All This is Brahman
All this is, indeed, Brahman.
This Atman is Brahman.
This same Atman has four quarters. (1.2)
[The first three quarters correspond to the letters of AUM: 1 = A, 2 = U, 3 = M. The fourth quarter is silence.]
"A" — Waking State
The first quarter is called Vaisvanara, whose sphere of activity is the waking state, who is conscious of external objects, who has seven limbs and nineteen mouths and who is the experiencer of gross objects. (1.3)
"U" — Dream State
The second quarter is Taijasa, whose sphere of activity is the dream state, who is conscious of internal objects, who is endowed with seven limbs and nineteen mouths and who is the experiencer of subtle objects. (1.4)
"M" — Deep Sleep
That is the state of deep sleep wherein one asleep neither desires any object nor sees any dream. The third quarter is Prajna, whose sphere is deep sleep, in whom all experiences become unified, who is, verily, a mass of consciousness, who is full of bliss and experiences bliss and who is the door leading to the knowledge of dreaming and waking. (1.5)
He is the Lord of all. He is the knower of all. He is the inner controller. He is the source of all; for from him all beings originate and in him they finally disappear. (1.6)
Gaudapada Karika Explanation (Ch 1)
[Gaudapada (c. 8th century CE) was first philosopher of the Advaita Vedanta school of Hindu philosophy in the post-Upanishadic period. He was guru of the great teacher Adi Shankara.]
Visva is all—pervading, the experiencer of external objects. Taijasa is the cognizer of internal objects. Prajna is a mass of consciousness. It is one alone that is thus known in the three states.
Visva is the cognizer through the right eye; Taijasa is the cognizer through the mind within; Prajna is the akasa in the heart. Therefore the one Atman is perceived threefold in the same body.
Visva experiences the gross; Taijasa, the subtle; and Prajna, the blissful. Know these to be the threefold experience. The gross object satisfies Visva; the subtle, Taijasa; and the blissful, Prajna. Know these to be the threefold satisfaction.
The experiencer and the objects of experience associated with the three states have been described. He who knows these both does not become attached to objects though enjoying them.
Surely a coming into existence must be predicated of all positive entities that exist. Prana manifests all inanimate objects. The Purusha manifests the conscious beings in their manifold forms.
Some of those who contemplate the process of creation regard it as the manifestation of God’s powers; others imagine creation to be like dreams and illusions.
Those who are convinced about the reality of manifested objects ascribe the manifestation solely to God’s will, while those who speculate about time regard time as the creator of things.
Some say that the manifestation is or the purpose of God’s enjoyment, while others attribute it to His division. But it is the very nature of the effulgent Being. What desire is possible for Him who is the fulfillment of all desires? (GK1.1-9)
Turiya 'the fourth' — Pure Consciousness
Turiya is not that which is conscious of the inner (subjective) world, nor that which is conscious of the outer (objective) world, nor that which is conscious of both, nor that which is a mass of consciousness. It is not simple consciousness nor is It unconsciousness. It is unperceived, unrelated, incomprehensible, uninferable, unthinkable and indescribable. The essence of the Consciousness manifesting as the self in the three states, It is the cessation of all phenomena; It is all peace, all bliss and non—dual. This is what is known as the Fourth (Turiya). This is Atman and this has to be realized. (1.7)
Gaudapada Karika Explanation
Turiya, the changeless Ruler, is capable of destroying all miseries. All other entities being unreal, the non—dual Turiya alone is known as effulgent and all—pervading.
Visva and Taijasa are conditioned by cause and effect. Prajna is conditioned by cause alone. Neither cause nor effect exists in Turiya.
Prajna does not know anything of self or non—self, of truth or untruth. But Turiya is ever existent and all—seeing.
Non—cognition of duality is common to both Prajna and Turiya. But Prajna is associated with sleep in the form of cause and this sleep does not exist in Turiya.
The first two, Visva and Taijasa, are associated with dreaming and sleep respectively; Prajna, with Sleep bereft of dreams. Knowers of Brahman see neither sleep nor dreams in Turiya.
Dreaming is the wrong cognition and sleep the non—cognition, of Reality. When the erroneous knowledge in these two is destroyed, Turiya is realized.
When the jiva, asleep under the influence of beginningless maya, is awakened, it then realizes birthless, sleepless and dreamless Non—duality.
If the phenomenal universe were real, then certainly it would disappear. The universe of duality which is cognized is mere illusion (maya); Non—duality alone is the Supreme Reality.
If anyone imagines illusory ideas such as the teacher, the taught and the scriptures, then they will disappear. These ideas are for the purpose of instruction. Duality ceases to exist when Reality is known. (GK1.10-18)
A + U + M+Silence
The same Atman explained before as being endowed with four quarters is now described from the standpoint of the syllable AUM. AUM, too, divided into parts, is viewed from the standpoint of letters. The quarters of Atman are the same as the letters of AUM and the letters are the same as the quarters. The letters are A, U and M. (1.8)
Vaisvanara Atman, whose sphere of activity is the waking state, is A, the first letter of AUM, on account of his all— pervasiveness or on account of his being the first. He who knows this obtains all desires and becomes first among the great. (1.9)
Taijasa Atman, whose sphere of activity is the dream state, is U, the second letter of AUM, on account of his superiority or intermediateness. He who knows this attains a superior knowledge, receives equal treatment from all and finds in his family no one ignorant of Brahman. (1.10)
Prajna Atman, whose sphere is deep sleep, is M, the third letter of AUM, because both are the measure and also because in them all become one. He who knows this is able to measure all and also comprehends all within himself. (1.11)
The Fourth (Turiya) is without parts and without relationship; It is the cessation of phenomena; It is all good and non—dual. This AUM is verily Atman. He who knows this merges his self in Atman—yea, he who knows this. (1.12)
Commentary by Shankaracharya:
Those who have realized Brahman, the Highest Reality, merge the self in Turiya because they have transcended the notion of cause and effect, which inheres in the third quarter of Atman. They are not born again; for they have realized their identity with the causeless Turiya. The illusory snake which has merged in the rope as a result of discrimination between the snake and the rope, does not reappear. Students of dull or mediocre mind who have renounced the world and are endowed with spiritual virtues should meditate on the common features of the sounds of AUM and the quarters of Atman, as explained before. Thus, proceeding step by step, they ultimately realize Turiya, devoid of any state or sound, and attain the Highest Goal.
Gaudapada Karika Explanation:
AUM should be known quarter by quarter. There is no doubt that the quarters are the same as the letters. Having understood AUM quarter by quarter, one should not think of anything else.
The mind should be concentrated on Aum. Aum is the fearless Brahman. He who is always absorbed in Aum knows no fear whatever.
Aum is verily the Lower Brahman. It is also stated to be the Higher Brahman. Aum is beginningless and unique. There is nothing outside it. It is unrelated to any effect and is immutable.
Aum is, indeed, the beginning, middle, and end of all things. He who has realized Aum as immutable immediately attains the Supreme Reality.
Know Aum to be Isvara, ever present in the hearts of all. The calm soul, contemplating Aum as all-pervading, does not grieve.
One who knows Aum, which is soundless and also endowed with infinite sounds, which is all good and the negation of duality, is a real sage, and none other. (GK 1.24-29)
* Commentary by Swami Nikhilananda