Uddālaka Āruṇi said to his son Śvetaketu - ‘O Somya, let me explain to you the concept of deep sleep. When a person is said to be sleeping, O Somya, he becomes one with Sat (Existence), and he attains his real Self. That is why people say about him, “He is sleeping.” He is then in his Self’.
Just as a bird tied to a rope flutter here and there, and when it cannot get any shelter anywhere, it surrenders itself to its bondage; in the same way, O Somya, the mind runs in every direction, and when it fails to get a resting place anywhere, it surrenders itself to prāṇa, the vital force. The mind, O Somya, is tied to prāṇa.
O Somya, now learn from me about hunger and thirst. When a person is said to be hungry, it is to be understood that the food he ate has been earned away by water. Just as people refer to a leader of cows, or a leader of horses, or a leader of people, similarly, people say that water is the leader of food. So also, O Somya, know that this sprout (i.e., the body) is the product of something (i.e., of food and drink). It cannot be without a root.
Where else, except in food, can the body have its root? In the same way, O Somya, when food is the sprout, search for water as the root; when water is the sprout, O Somya, search for fire as the root; when fire is the sprout, O Somya, search for Sat (Existence) as the root. O Somya, Sat is the root, Sat is the abode, and Sat is the support of all these beings.
Then when a person is said to be thirsty, it is to be understood that the water he drank has been carried away by fire. Just as people refer to a leader of cows, or a leader of horses, or a leader of people, similarly, people say that fire is the leader of water. So also, O Somya, know that this sprout (i.e., the body) is the product of something (i.e., of food and drink). It cannot be without a root.
Where else, except in water, can the body have its root? O Somya, when water is the sprout, search for fire as the root; when fire is the sprout, O Somya, search for Sat (Existence) as the root. O Somya, Sat is the root, Sat is the abode, and Sat is the support of all these beings. As to how, O Somya, these three deities (fire, water, and earth) enter a body and each becomes threefold, this has already been explained. O Somya, as this person is dying, his speech merges into the mind, his mind into prāṇa, his prāṇa into fire, and then fire merges into Brahman, the Supreme Deity.
‘That which is the subtlest of all is the Self of all this. It is the Truth. It is the Self. That you are, O Śvetaketu.’ Śvetaketu then said - ‘Sir, please explain this to me again.’ ‘Yes, Somya, I will explain again,’ replied his father.