From the Prasnopanishad of the Atharva Veda
The setting: A group of six earnest seekers after truth, all of them earnest students of philosophy, gathered together, only to find that each one of them had a special problem to solve, and that none of the others could solve it for them. They decided by common consent to approach the sage Pippalada and have their problems solved. After a year of living with the sage at his ashram, meditating and practicing brahmacharya, it was finally time to for them pose their unresolved question to the learned sage.
…It was now the turn of Gargi Souryayani to avail himself of the opportunity to have his problems solved, and to proceed in a spiritual sense from darkness to light. He raised a metaphysical problem, which is peculiar to Indian philosophy. Ordinarily speaking we see the various levels of consciousness. He wished to know who it is that experiences these levels of consciousness.
This is a problem that every seeker after truth encounters. Perhaps Sage Pippalada had answered the same question to previous seekers after the truth who had come to him to know the nature of the Akshara-purush.
Sage Pippalada illumined the problem in his characteristic way.
He said, “During the waking stage we think waking alone is real; during a dream, the dream alone appears real; and during deep sleep, there seems to be nothing else. If we reduce all these levels of consciousness to their least common factor, we find that the experiencing Self alone is real. Waking goes away, dream goes away, sleep goes away but the Self alone remains, and knows the Self that has slept, or dreamt or is awake.
“Each stage is of value to us. The dream world is of value to us because it shows us how a whole world of reality can be a mere projection of the mind, and (when it ends) can be withdrawn into the mind. The sleep consciousness is of value to us, because it is, in a sense, the homecoming of the spirit - it is a withdrawal and merging into that from which it comes. Although in sleep one is aware of nothing yet one emerges from it with the consciousness “I have slept well”. Waking is of value to us because it is here that we should know the nature of that ‘I’. We would then realize that it is the residuary value of life. It is only that which gives value to the other levels of consciousness.
“It is like the Sun whose light reveals objects. Suppose there are no objects for the Sun to shine upon. Even then the Sun will continue to shine. It is the same with the Self also. Levels of consciousness may come and go, but the Self goes on forever. That is why, taken in itself, it is called the turiya, or the fourth level of consciousness. It is not something that is apart from these three. It is, if you choose to call it, the transcendent. In reality, it is the Akshara-purush, Brahman, who is the true seer, actor, knower, the undecaying eternal Self. That alone is all-pervading.
On hearing what the preceptor had to say, Gargi felt happy as if he had acquired the kingdom of knowledge.
- from Upanishadic Stories And Their Significance by Swami Tattwananda, published by Ramakrishna Advaita Ashrama