The life of a human being encompasses all that he or she can experience in this lifetime. A life driven by the senses and desires runs after sense objects: objects of desire which can be wealth, power, or hedonism.
Somewhere between this drama of life, we take a pause as we become tired of the rat race and reflect fleetingly on what it is that we are running after? When this pause becomes more frequent, we turn our sense of perception from the outside to inside as we realise that each and every moment we are moving closer to the pyre and have very little time on earth to unearth the secret of our being.
Our desires are never fulfilled by external objects, thus a sense of incompleteness engulfs us and we want to expand our horizon and gaze onto an unchartered pathway, the territory of spirituality. This is the ultimate fate of each human being on earth. The only difference is the time at which one feels this urge to know the limitless. Thus, the first line of Patanjali Yoga sutra starts with the sutra  “Atha yoga anusashanam.” ‘Atha’ means now. It is a very powerful word. It is the present moment, the ‘Now’, that really matters. We have this moment, this ‘Now’, to be utilised fully. If only we were to look deeply, that the mystery of the universe would unfold within us. This can happen ‘Now’, i.e. atha, at this very moment, we only need the will.
When one wants to move into the spiritual path, the sense of perception needs to be refined. This is possible by purification of the heart through disciplines such as control of the senses, concentration, performing one’s rites or duties combined with devotion, being dispassionate, and free of sin or deceit. The secret of moving from a limited entity bound by our physicality and mentality to boundless limitless formless pure consciousness is revealed only to those who have made necessary preparation, and have prepared the soil before sowing the seed.
This knowledge of reality or the highest truth or Atman or Brahman, whatever one calls it, is eternally existent. It is obscured by the ignorance in the Jiva. The aim of Sadhana is to remove this obstruction, to break this wall.
We become eligible for knowledge of Atma (Brahma Vidya) only when we understand the relationship between Brahma and humanity in the cosmic cycle of creation and destruction. ‘Brahma’ means the vast, infinite principle. The element of sound (Akasa) emanated from this infinite expanse of Brahma. From sound emerged air (Vayu), fire (Tejas) from air, wind from fire, earth (Prithvi) from wind, oceans from earth, food from oceans, and finally, humans from food. Ponder this sequential cycle and you will arrive at the truth that humanity has emerged from the infinite Brahma.

The purpose of enquiry

Now the question that arises in our minds is that why should I learn about the nature of reality, why should I inquire? How would it help me in my day to day life? The answer given by Vedanta is when we realise the truth, our fundamental outlook on how we look at life changes, and so we can have a better handle on our life. We have two ways of enjoying a theatrical show. The actors who take part in it and the spectators, both enjoy the show. The actors identify themselves with the characters and take the show as real. Therefore, they cannot enjoy the show in reality. But the spectators know the unreality of the show and are detached from it, and so they really enjoy it. So, we have two ways to look at life. When we are spectators or witness to this drama of life, we will really enjoy it. In the language of Shankara, the knowledge of reality cuts through our ignorance and dehypnotises our mind so that it destroys our hankering for objects which are unreal. The elimination of ego is the basic requirement for acquiring any knowledge. When a glass is empty you can fill it. If we pour water in glass which is full,  it will only overflow. So, the first step is to make our conditioned mind empty of all preconception and eliminate our ego. The whole of life is thus an ongoing process or, yoga, or discipline or path to realise the truth. The goal is as important as the way, the path. The Sadhana Chatusthay for this is discrimination, renunciation, self-control and an irrepressible urge for the realisation of truth.
Thus, our sense of perception is the doorway to this mystery of our being. If we closely observe our lives, we find that we move seamlessly between three states of awakening, dream,  and deep sleep on a daily basis. The well-known method of Vedanta to arrive at reality is what is known as “vichara” or contemplation and discrimination. In Mandukya Upanishad, we find the problem of handling the ultimate reality straightforward. Thus, it goes on to illustrate the three states of our existence and also posits a fourth state and its significance. The experiencer of the three states, also known as ‘Viswa’, ’Taijasa’, and ‘Prajna’, or,  Waking (Wakefulness), Dream and Deep Sleep is only one. The fourth, or the ‘Turiya’ is the Atman or Brahman.
Now let us explore in detail the three states of our being and inquire about the fourth state in the light of the Mandukya Upanishad with Goudapada’s Karika and commentary of Shankara. Goudapada lays emphasis on the fact that it is impossible to reach the highest truth until and unless the totality of human experience or knowledge can be taken into consideration. The Upanishads show us that unless the three states of Waking, Dream and Deep Sleep are coordinated, there cannot be adequate data for the inquiry of the ultimate truth. According to Goudapadacharya and Shankaracharya, philosophy is an interpretation of the totality of human experience, or of the whole of life from the standpoint of truth. It does not only relate to the waking state. Gourapadacharya was the Guru of Govindacharya who was the Guru of Shankaracharya. In that sense, Goudapadacharya was the great-grand-Guru of Shankaracharya.

Brahman is Aum

The phenomena of the world consists of ideas or the mental states. The ideas can be expressed as words. The utterance of the word AUM is the clue to the pronunciation of all the words used by human beings. It is the substratum of all the sounds. As“Sabda Brahman, Brahman is also the substratum of all objects, animate or inanimate and is all pervading. Not only this, it is the substratum of all phenomena. The sound signifying the phenomena is non-different from the phenomena. Hence, Brahman is AUM. The AUM which represents the phenomena of Brahman has three mantras. A (The waking state), U (The dream state) and M (The deep sleep state). It has one Amatra, which is the silence after AUM. This is the fourth, The Turiya. This Turiya is Brahman.
In Kathopanishad, it is mentioned that when Aum is uttered with concentration, there arises the consciousness of Brahman in the mind. Through the meditation on AUM, one can realise both the para (attributeless) Brahman and apara (name and forms) Brahman. One who seeks to realise the self with one point concentration on Aum feels that the gross universe (A) is absorbed into the subtle (U) and (U) then into the causal (M), and finally the universe, dependent on the causal relation is withdrawn into the transcendental, which is known as Amatra, and which cannot be designated by any letter or sound. It is this stillness which is ‘the fourth’, the Turiya or The Brahman.

The Upanishads elaborate on the principle of Brahman through three names: Viraat, Hiranyagarbha, and Avyaakruta or Iswara. These three forms relate to Gross(Sthulam), Subtle (Sukshmam), and Causal (Kaaranam) and to the three states of  Waking, Dream, and Deep Sleep. The transcendental Brahman or the Turiya i.e. The Fourth is beyond all causal relations and is the ultimate substratum of all appearances i.e. Virat, Hiranyagarbha and Iswara.
In Agama Prakarana, the first chapter of Mandukya Upanishad, it is said
”All this is verily Brahman. This Atman is Brahman. This Atman has four quarters or four padas.”
These four padas are not like the legs of cows which are unrelated to each other. These four padas are the quarters of a coin or Karshapana. The quarter is merged in the half coin. The half is merged into three fourths and the three-fourths is merged in the full. So the Viswa or Waking State is merged in Taijasa, The Dream State and Taijasa are merged in Prajna or Deep Sleep State. Finally, Prajna is merged in Turiya, The Fourth. So, the Fourth or Turiya is realized by merging the three states in it. This fourth state or Turiya is the object of enquiry.

The four Padas or quarters

1. The first quarter is the Viswa, the Waking State: It is said in the Mandukya Upanishad that,

“The first pada is Vaiswanara, whose sphere of activity is the waking state, who is conscious of external objects, who has seven limbs and nineteen mouths, and whose experience consists of gross objects.”

His sphere of activity is the Jagritsthana or Waking State. Because of ignorance or Avidya, consciousness appears as related to outward objects other than himself. So he is Bahishprajna. He is Saptanga i.e., having seven limbs in his body. “Seven limbs”: The word ‘limbs’ is used here to denote parts of the body. The seven limbs are The Head, The Eyes, The Mouth, The Breath, The Middle Part of the Body, The Kidney, and The Feet. They have their counterparts in the universe, namely, The Heavens, The Sun, Fire, Air, Akasa (space), Water, and Earth. He has nineteen mouths, which means, five organs of perceptions (Buddhindriyas), five organs of actions (Karmendriyas), five aspects of vital breath (Prana, etc), the mind (Manas), the intellect (Buddhi), egoity (ahankara), mind-stuff(Chitta). These are the instruments by which Vaiswanara experiences objects. He is non-different from the totality of gross bodies or Virat. He is the first quarter, as the subsequent quarters are realized through him. He is called Vaiswanara,i.e. All Beings since he encompasses all beings by virtue of his being non-different (in reality) from the self i.e. Virat comprising all the gross bodies.
The Parallelism Between Microcosmic And Macrocosmic
He exists in everyone but is undetected. He performs all actions but behaves as if He does not. He experiences everything but does not reveal it. Therefore, Viraat is the principle that manifests as the external, gross, visible world. The universe may be seen from two standpoints: The Microcosmic and The Macrocosmic. The Microcosmic (subjective) entity (Adhyatma) is endowed with four quarters, namely, Vaisvanara (or Visva), Taijasa, Prajna, and Turiya. Likewise, the macrocosmic (objective) universe, comprising the sphere of the sun, the moon, the stars, etc., has four quarters. The first three are known as Virat (the totality of gross physical bodies), Hiranyagarbha (the totality of subtle bodies), and Isvara or Avyakrita, the unmanifested (the totality of causal bodies). The attributeless Brahman, like Turiya, is the fourth. It is transcendental, beyond all causal relations, and is the unrelated substratum of all appearances. A parallelism runs through the subjective and the objective. The macrocosm is superimposed upon Brahman, and the microcosm upon Atman (Self), through Avidya (ignorance). Both are illusory appearances. On account of the non-difference between the subjective and the objective, the limbs of Vaisvanara are described in terms of the objective universe. The purpose is to show the illusory nature of the entire phenomenal world and establish the non-duality of Atman (Self) and Brahman (Supreme Reality).
2. The second quarter is Taijasa or The Dream State: It is the second quarter and its sphere of activity is the dream state whose consciousness is internal, who is possessed of seven limbs and nineteen mouths, and also enjoys subtle objects.”
Thus, the sphere of activity is Swapnasthanah. The waking consciousness as it is associated with many means and being conscious of objects as if external, are nothing but states of mind, leaves in the mind corresponding impressions. Under the illusion of ignorance, desire, and past action, the mind thus possessed of the impressions like a piece of painted canvas makes its appearance in The Dream State just as in the waking state but without any external means. The mind is aware of internal objects or Antah (internal) Prajna ( awareness).  Here the enjoyment is subtle. He is called Taijasa or luminous  since he becomes the witness of the cognition that is bereft of objects and appears only as a luminous thing. The object of experience is consciousness consisting of Vasanas, Ie impressions of past experiences.
The second form of Brahman is Hiranyagarbha: the basis, the source of all wisdom. All knowledge —worldly, ethical, dharmic, spiritual, scientific— has emanated from Hiranyagarbha. Therefore, He is also called the Sun of Knowledge (Jnana Bhaskara). When the Sun rises, He appears golden (Hiranya) and colours the world with a golden hue.
Hiranyagarbha is a Sanskrit term that translates to ‘golden embryo’,’golden womb’ or ’golden egg’. It is derived from the root words ‘hiranya’, meaning golden or, wealth, and ‘garbh’a, meaning womb, germ/seed or essence. Creation emerged from Hiranyagarbha, which is oval in shape like an egg. The mouth was the first to emerge from this shape. From the mouth, sound. Then, the nose, through which air began flowing. Eyes came next, with fire emanating through them. Ears arrived next. Therefore, Hiranyagarbha is the origin of the human form. Hiranyagarbha, like Viraat, has two other names: Sutraatmaka and Praana. Just as a thread (sutra) passes through a necklace of gems, Sutraatmaka is the principle that underlies all beings and brings bliss to them. This unifying thread-like principle is also called Brahma Sutra.
Hiranyagarbha assumes a subtle (sukshmam) form during the dream state —the form of the Atma. This means that Hiranyagarbha is one who is awake during the dream state. Viraat creates everything in the waking state. Hiranyagarbha creates everything in the dream state by mere willing. Every object is created by will alone. Whatever is seen in dreams is created by Hiranyagarbha.
The reflection of the body (Deham) is the external world (Desam).

 “Outside is a reflection of the inner Being.”

3. The third quarter is the Prajna or The Deep Sleep State: That state is Deep Sleep where the asleep individual does not desire anything and hence does not see any dream. The third state is called Prajna i.e. where all experiences become undifferentiated, which is a mass of mere consciousness, which abounds in bliss, where one is surely an enjoyer of bliss, and who is the doorway to the experience of the two other states of waking and dream.”
The sphere of the Deep Sleep state is Susuptasthana. He is said to be eki bhutah i.e. undifferentiated. Conscious experiences that are but vibrations of the mind in the waking and dream states, become as they were. This state is called Prajnanaghanah, a mass of consciousness, since it is characterised by the absence of discrimination. The word ‘eva’ denotes verily i.e. the absence of any other thing except consciousness. He is Anandamayah, full of joy, his abundance of joy being caused by the absence of misery involved in the effort of the mind vibrating as the objects and their experiencer. But he is not bliss itself as the joy is not absolute and the bliss is not infinite. He is Anandabhuk, the experiencer of joy. What is common to all the three states is the absence of the knowledge of reality. He is Cetomukhah, since he is the doorway to the consciousness of the experiences in the dream and waking states.
The third form of Brahman is Avyaakruta. He is absolutely formless. He exists in the causal body and enjoys the state of deep sleep. He has no limbs but does all actions. He travels to distant places.  He has no eyes but sees everything. He has no ears but listens to everything. He performs the act in Creation, but possesses no form.  He also has two more names:
Antaraatma and Iswara. Antaratma is the inner voice as all inspiration, urge and every motive comes from this antaratma. Iswara is the embodiment of all forms of  prosperity (Sakala Aisarwa Swarupa)
Jagrat, Swapna and Sushupti are the three states where the perceiver experiences the three states. The common term used for them is Supta which denotes the absence of knowledge of reality. ‘Sushupti’ is designated as the state of causal unity of waking and dream state from the stand point of a waking man. But this is also a ‘vritti’ in the mind of a waking man to find the cause of waking and dream state. The perceiver in the ‘jagrat’ and ‘swapna’ states, who always experiences subject-object relationship, finds its absence in Sushupti.
Atman The Witness
Goudapada says, “It is only one alone who is thus known in the three states.” It denotes the Atman (the witness) is distinct from these three states (i.e. the witnessed). As a powerful fish swims from one bank of the river to the other unimpeded by the currents of the river, so also Atman moves in the three states totally unaffected by them. No experiences of the three states affect the pure Atman.
4. The Fourth state TURIYA: It is sought to be indicated by the negation of all attributes – Brahman appears as the world without forfeiting its essential nature just like the classic Advaitic example of a rope that appears to be a snake. The fourth is not the fourth state or condition in which Atman has to be viewed. Turiya which is indicated here as the “fourth” and which actually means the fourth step, comes into consideration only when all the other three states have been considered. Atman itself does not admit of any condition or state. Waking, dream and deep sleep are its three states or quarters, but Turiya, the fourth is actually present in all these three states.

Thus, ultimately, it is said that Duality cannot exist, when Gnosis , the highest truth (Non duality) is realised. It is shown that the meaning of the  Vedic statement,

“That thou art” points to the relationless Atman (Turiya), which is like the rope, different from the snake.

The Way To Know Turiya, The Fourth

Goudapadacharya, according to the scriptures, prescribes Asparsa Yoga as the means to realise this Turiya. The term ‘”Äsparsa Yoga” means “yoga of non-contact”. Here, ‘yoga’ is not used in the sense of union, but as discipline or path. The word ‘asparsa‘ denotes freedom from relationships and refers to the Non-Dual Brahman alone. So, the Asparsa Yoga is the path or discipline which leads to the freedom from relationships or non-dual Brahman. This alone is beneficial to all beings and can bring joy to all.
References:
1. Eight Upanishads, Vol Ii, Swami Gambhirananda
2. Mandukyoponishad With Karika, Swami Nikhilananda