The Upanishads, written down ca. 700-200 BC, are considered the heart of Indian philosophy. They belong to the ancient texts of Hinduism and are widespread and much-recited, even today. Most of them are part of the Vedas, from the four collections forming the earliest body of Indian scripture, consisting of the Rig Veda, Sama Veda, Yajur Veda, and Atharva Veda.
In the Tattiriya-Upanishad, Part II.3.1 we read: The spiritual body of man consists of "Yahur-Veda as the head, Rig-Veda as the right arm, Sama-Veda is the left arm, the Upanishads are the heart, and Athar-Veda is the foundation.
The Upanishads are a huge and profound collection of explanations and dialogues, for the most part between a master and his disciple but also between kings and sages. They convey a multitude of spiritual topics contemplating the universal philosophy and centered on how to overcome the circle of birth and death from different angles and in different contexts.
The Chandogya Upanishad introduces us to the wisdom of Shandilya in a few words:
All this is Brahman. From It the universe comes forth, in It the universe merges and in It the universe breathes. Therefore a man should meditate on Brahman with a calm mind. Now, verily, a man consists of will. As he wills in this world, so does he become when he has departed hence. Let him with this knowledge in mind form his will.
He is my Self within the heart, smaller than a grain of rice, smaller than a grain of barley, smaller than a mustard seed, smaller than a grain of millet; He is my Self within the heart, greater than the mid-region, greater than heaven, greater than all these worlds.
He whose creation is this universe, who cherishes all desires, who contains all odours, who is endowed with all tastes, who embraces all this, who never speaks and who is without longing −He is my Self within the heart, He is that Brahman. When I shall have departed hence I shall certainly reach Him: one who has this faith and has no doubt will certainly attain to that Godhead. Thus said Sandilya, yea, thus he said. (Part 3, 14. Khanda)
For millennia humanity has been taught that the searching outside of us cannot deliver us. Only "turning around" and searching within ourselves enables us to overcome and resolve the opposites that are so prominent and so powerful in the outside world. If we can unite Atman—, our innermost Self—with Brahman—the creator of the universe—then there is no more separation or duality, no alternation between life and death.
Thus, the Prasshna Upanishad (question VI) states:
As these flowing rivers, bound for the ocean, disappear into the ocean after having reached it, their names and forms being destroyed and are called simply the ocean−even so, these sixteen parts1 of the seer, whose goal is the Purusha2, disappear into the Purusha after having reached Him, their names and forms being destroyed and are called simply the Purusha. He becomes free of parts and immortal. On this there is the following verse :
Know Him, the Purusha, who alone is to be known and in whom the parts rest firm, like the spokes in the hub of a wheel, that death may not affect you
Pippalada said to them: Thus far, indeed, I know the Supreme Brahman; there is nothing higher than this.
The hub of the wheel, where all manifestations become one, and the source from which they emanate, is The Self whose realisation leads to a state of consciousness that is "not of this world". It is like the omniscience of the ocean into which the rivers flow, giving up their individuality. Next to this state of consciousness, all manifestations - including the wheel of birth and death - pale.
The Upanishads belong to the great testimonies concerning humanity's path, the path that will be completed upon the completion of self-recognition. "Man, know thyself!" – this command that crowned the portal of the Temple at Delphi is the universal star that guides the entire path.
Seek thy true self and unite with it!