Katha Upanishad found in the Vedas (Yajur Veda) contains the all important teaching in Hinduism. The teaching is narrated by Yama, the God of Death, to Nachiketa, a young boy.
Nachiketa asks the question – What happens to a man after death? Does the soul exist or not? Yama tries to dissuade the boy from pursuing the question by promising enormous wealth and power in lieu to an answer. The boy stands firm and sticks to his boon, saying that he does not care for material prosperity, which is transient by nature. From this point, the actual teachings of the Upanishad begin to unfold in their entire splendor.
There are two paths open to every man – the path of knowledge and the path of pleasure. The latter leads to some kind of joy, however, it is temporary; it leads only to bondage and loses its value in course of time. A wise man should forsake this path and take to the path of knowledge, with the help of a proper teacher. He should relinquish both joy and sorrow and meditate on the soul (atman), which is subtle, immanent, and all pervading. Atman is neither born nor dead. It has not come into being from anything nor has anything come into being from it. This unborn, eternal, everlasting, ancient one suffers no destruction even when the body is destroyed. It is smaller than the smallest and greater than the greatest. It dwells in the hearts of all creatures. The desireless one, being free from grief, realizes the glory of the atman through the purity of the senses and the mind.
The atman is the master of the chariot, who sits within it; the body is the chariot, the intellect is the charioteer, and the mind is the reins. The senses are the horses, and the roads are the sense objects. The wise regards the inner self as the witness to all actions of the body, the senses and the mind.
A wise man should control the sense organs just as a charioteer controls the horses. One who lets the sense organs wander at will get into the never-ending cycle of births and deaths. The objects are superior to the senses since they attract them; the mind is superior to the objects; the intellect is superior to the mind; and the individual self is superior to the intellect. The primal unmanifest matter is superior to the individual self, and the universal self superior to them all.
The atman is all pervading, yet few realize its presence. The wise should merge speech in mind, mind in the intellect, intellect in the individual self, and the individual self in the universal self.
The individual self merges into the universal self and becomes one homogeneous entity, like pure water poured into pure water. The individual self is not essentially different from the universal self. As the same air assumes different forms and names (as wind, gale, hurricane, etc.), so the one atman that abides in the hearts of all beings appears as having different forms according to the different objects it enters, and it exists beyond them also. The atman is the cause of all creation, yet it is not different from them. It is seen in the self just as one sees oneself in the mirror. Those who realize the truth become immortal. So Upanishad states – Arise, awake, and having approached the wise become realized (uttishthata jagrata, prapya varan nibodhata).