kaṭhopaniṣad - adhyāya 2, vallī 3

This ancient Ashvattha tree has its roots above and its branches below. That alone is the Pure, that is Brahman, and that alone is called the Immortal. All worlds are supported in It, and none can transcend It. This, verily, is That.

Whatever exists in this universe arises from Brahman and vibrates in the Prana. It is a great terror, like a raised thunderbolt. Those who know this become immortal.

Out of fear of Him, Agni (fire) burns; out of fear, the Sun shines; out of fear, Indra, Vayu, and Death, the fifth, hasten to perform their duties.

If one is able to realize Him here on earth before the dissolution of the body, one will be liberated from the bondage of the world; if not, one must again take on a body in the worlds of creation.

As in a mirror, so in the Self; as in a dream, so in the world of the ancestors; as in water, so in the world of the Gandharvas, the Self is seen as reflected; as light and shadow, so it is in the world of Brahman.

The wise man, having realized that the senses arise and perish separately, and that they are distinct from the Self, ceases to grieve.

Higher than the senses is the mind; higher than the mind is the intellect (Sattva); higher than the intellect is the Great Self (Mahat); higher than the Great Self is the Unmanifest (Avyakta).

Higher than the Unmanifest is the Purusha (the Supreme Being), who is all-pervading and devoid of any mark or attribute. By realizing Him, a living being is liberated and attains immortality.

His form is not within the range of vision; no one can see Him with the physical eye. He is revealed by the heart, by the intellect, and by the mind. Those who know this become immortal.

When the five senses of perception are stilled together with the mind, and the intellect does not waver—that state is called the Supreme Goal.

This steady control of the senses is what the wise call Yoga. One must then be vigilant, for Yoga is both the origin and the dissolution of all things.

The Absolute cannot be reached by speech, by the mind, or by the eye. How can It be known by anyone other than him who speaks of It as existing?

It should be apprehended as the Existence that underlies both aspects. When It is apprehended as Existence, Its true nature reveals itself.

When all desires dwelling in the heart are cast away, then the mortal becomes immortal, and in this very body, he attains Brahman.

When all the knots of the heart are severed in this life, then the mortal becomes immortal. This is the entire teaching.

There are a hundred and one channels of the heart, and one of them pierces the crown of the head. By going upward through this channel, one attains immortality; the others lead to departure in various directions.

The Purusha, no larger than a thumb, the inner Self, is always seated in the hearts of all men. One should draw Him out from one’s own body with firmness, like the pith from a reed. Know Him to be pure and immortal; know Him to be pure and immortal.

Having received this knowledge taught by the Lord of Death, and the entire process of Yoga, Nachiketa became free from impurity and death, and attained Brahman. So too will any other who thus realizes the inner Self.