The first two states are not true experiences of reality and truth because of their dualistic natures of subject and object, self and not-self, ego and non-ego. In the third state, dreamless sleep, one is not conscious of external or internal objects; however, that does not mean consciousness is not present there. It is like saying 'I don't see anything in darkness'. The recognition that I don't see anything is what I 'see'. So also in dreamless sleep, one is not conscious of anything and the very fact that this statement is true proves the existence of consciousness during deep sleep.
Consciousness is the constant factor in all the three states, and it is unaffected by the presence or absence of objects. Consciousness itself does not require to be revealed by another consciousness, as it is self-revealed. While everything is presented to consciousness and is revealed by it, consciousness itself is not presented to anything else. It is never an object in relation to another subject. It is that which underlies both subject and object. It is the fourth, the turiya, the brahman. The Mandukya Upanishad defines turiya as:
"The fourth state is not that which is conscious of the subjective, nor that which is conscious of the objective, nor that which is conscious of both, nor that which is simple consciousness, nor that which is all-sentient mass, nor that which is all darkness. It is unseen, transcendent, the sole essence of the consciousness of self, the completion of the world."
Turiya represents consciousness free from material influence. The idea is that consciousness, of which the atman is constituted, exists in our wakeful state of material experience, as it continues during sleep. In sleep we dream and experience the mental realm, whereas during our waking state the physical plane has more bearing on our lives.
Upon awakening from deep dreamless sleep, one remembers existing in that condition. This is evidenced by the common expression, 'I slept well!' One cannot remember something one has no experience of.
Thus, in deep sleep when intelligence is transformed by tamo guna, the self continues to exist, as it does when intelligence is transformed by rajo guna during the dream condition and during the wakeful condition when intellect is transformed by sattva guna. The self is independent of the body and mind. If the physical and mental realms were to shut down, the self would continue to exist. This we know from our experience in deep sleep. Realizing this involves entering the turiya.
"Within the material world the Lord appears as the three Visnus (gunas). The original form of the Lord is another form still. He is beyond material nature and thus known as the fourth."
The Gaudiya Vedantins are interested in turyatitah gopala (Lord Gopala beyond fourth dimension, Gopala Tapani Upanishad 2.96). This is the fifth dimension in which one comes face to face with Gopala Krishna in Braj (Vraja Dhama), from adhoksaja to aprakrta, or from God consciousness to Krishna consciousness.
"The fourth dimension, turiya, is the ground of our existence and the goal of all transcendentalists. For the Vedanta philosophers it is perceived variously, either as undifferentiated consciousness or a relationship with the divine. Regarding the latter, Gaudiya Vedanta concludes that love is greater than ourselves, and it is the greatest aspect of God, one that he himself is motivated by. For them, the nondual consciousness of Vedanta philosophy is realized when we know that we do not belong to ourselves, what to speak of anything belonging to us. If there is any time at which we can accurately say that something belongs to us, it is when, having given ourselves in love to God, we can say that 'he is ours'."
"This is the Krsna (Krishna) conception of Godhead, one in which God appears not as God, nor finite souls as finite souls. Both interrelate intimately as lover and beloved, Krsna and his gopis, beyond any sense of each others' ontological reality, yet beyond the material illusion as well. This dimension of love of Godhead is thus justifiably termed by the Gaudiya Vaisnavas as the fifth dimension, turiya-titah (turyatita or turyatitah), the dimension of the soul's Soul."