The sacred "OM."
   A -- U -- M --      
 

This represents 3
states:  awake, 
dreaming, and 
dreamless sleep.
It is followed by full
silence, which stands
for moksha --
salvation by release
from individual
existence. See Ch. 5]
 
 

There are four ancient Vedas, sacred texts derived for the most part from beliefs and practices of the Aryans who invaded India around 1500 B.C. in the time of archaic polytheism. The first of these four texts, the  Rig Veda, contains mostly hymns sung at sacrificial rituals in praise of the various gods. Other Vedas contain more hymns, instructions for ritual, and even recipes for magic. Traditionalist Hindus revere the Vedas as the work of rishis (seers or "sages") who were divinely inspired.

Some material in the Rig Veda was added more recently, though scholars do not agree on the dates. Some material in Book 10 of the Rig Veda originated, perhaps, in the 8th century BCE or later.  One of the clues that scholars use in assigning this date is the change in mode of expression and in content. In place of hymns and rituals and magical formula there are verses that are rather philosophical in style. In place of praise of the powerful gods and pleas for aid and mercy from them, there are abstract ideas about the Ultimate origin and Power behind all else. Hymn 129 of Book X is the most famous of these early historic style passages.  These passages are hard to understand, perhaps because they are among the earliest philosophical reflections about the Ultimate with little prior philosophy to guide them.  Even the scholars and translators disagree on what some of it means.  But note that the questions here intend to answer the ultimate questions about the origin of everything.

RIG VEDA, BOOK X, HYMN 129.⁽¹⁾

Then was not non-existent nor existent: 
    there was no realm of air, no sky beyond it.
What covered it, and where? and what gave shelter? 
Was water there, unfathomed depth of water?
Death was not then, nor was there aught immortal: 
    no sign was there, the day's and night's divider.
The one thing, breathless, breathed by its own nature: 
    apart from it was nothing whatsoever.
Darkness there was: 
    at first concealed in darkness this All was indiscriminate Chaos.
All that existed then was void and formless: 
    by the great power of Warmth⁽²⁾ was born that Unit. 
Thereafter rose Desire⁽³⁾ in the beginning, 
Desire, the primal seed and germ of Spirit.
Sages⁽⁴⁾ who searched with their heart's thought 
    discovered the existent's kinship in the non-existent.
Transversely was there severing line extended:⁽⁵⁾
    what was above it then, and what below it?
There were the begetters, there were mighty forces, 
     free action here and energy up yonder.
Who verily knows and who can here declare it, 
    whence it was born and whence comes this creation?
The Gods are later than this world's production. 
   Who knows then whence it first came into being?
He, the first origin of this creation, 
   whether he formed it all or did not form it.
Whose eye controls this world in highest heaven, 
    he verily knows it  --  or perhaps he knows not.

There is a large collection of works that are thought of by Hindus as "commentary" upon the Vedas. Some of them supposedly date as far back as 1,000 B.C., though other scholars claim that even the earliest of them was composed in the 8th century B.C. or more recently.  Of the 110 (or 118, depending on who is counting) there are 10 "principle Upanishads" (or 11 or 14, again depending). The "principle" ones were called this by even later commentators, who were most impressed precisely by those Upanishads which contained the most "historic" (or classical) type of abstract thought. These are the Upanishads which focus mainly on the Ultimate Self or Brahman and the means to attain union with the Ultimate through detachment from worldly concerns like wealth, pleasure, power, and status. 

Kena Upanishad:
First, here are a few lines from the Kena Upanishad (II,3-4), worth citing, because they echo the idea common to so much of historic or classical religious thought, that the Ultimate is beyond human comprehension and description.  Compare these to words from the Tao Te Ching.

It is conceived of by him by whom It is not conceived of.
He by whom It is conceived of, knows It not.
It is not understood by those who [say they] understand It.
It is understood by those who [say they] understand it not."⁽¹⁰⁾

The Katha Upanishad is one of the most famous of all Upanishads. It is world-rejecting, teaching how to get free of "the round of births" [perpetual reincarnation].  Standing at the transition from archaic reliance on stories alone, to historic use of abstract analysis, it combines the two approaches by telling a story in which Death teaches abstract ideas to the hero

KATHA UPANISHAD, 3:10-17 ⁽⁶⁾

This Upanishad tells a story, of the young man Nakiketas, whose father in a rash moment had promised him in sacrifice to Death. In the First Valli [chapter] Nakiketas enters into Death's house, where he impresses death with his noble character. So Death explains the meaning of some basic rituals, in particular the fire-sacrifice. Nakiketas is not satisfied with understanding traditional practices.  He presses on with deeper questions. What is the state of the hereafter? Is it existence or non-existence? What, in fact, is the goal of life? Nakiketas' demeanor and responses still impress Death, so he accedes to Nakiketas' desire for more information.

Death begins his answer in the "Second Valli" [chapter].  Death asserts the difference between ignorance and wisdom. The wise person recognizes that the Self [Atman] is the goal, not the transient self but the eternal Self. Nakiketas accepts all this. Death continues, now identifying the Self with Brahman, and declaring that the sacred syllable "OM" [or AUM--three sounds all together] is the way to affirm and accept Brahman and the Self.⁽⁷⁾

In the Third Valli, Death describes the basic orientation of the person who can attain to Self.  (The analogy of the person to a charioteer appears also in Plato's Republic.  No one seems to know whether Plato borrowed from this Upanisahd.)  The analogy gets difficult to follow at times.  Read it just for a general sense of its content.

3. `Know the Self to be sitting in the chariot, the body to be the chariot, the intellect (buddhi) the charioteer, and the mind the reins.'<sup>(8)</sup> 4. The senses they call the horses, the objects of the senses their roads. When he (the Highest Self) is in union with the body, the senses, and the mind, then wise people call him the Enjoyer.' 5.`He who has no understanding and whose mind (the reins) is never firmly held, his senses (horses) are unmanageable, like vicious horses of a charioteer.' 6. `But he who has understanding and whose mind is always firmly held, his senses are under control, like good horses of a charioteer.'

7. `He who has no understanding, who is unmindful and always impure, never reaches that place, but enters into the round of births.' 8.`But he who has understanding, who is mindful and always pure, reaches indeed that place, from whence he is not born again.' 9.`But he who has understanding for his charioteer, and who holds the reins of the mind, reaches the end of the journey. That is the highest place of Vishnu.'⁽⁹⁾

10. `Beyond the senses there are the objects, beyond the objects there is the mind, beyond the mind there is the intellect, the Great Self is beyond the intellect.' 11.`Beyond the Great Self there is the Undeveloped, beyond the Undeveloped there is the Person (perusha). Beyond the Person there is nothing--this is the goal, the highest road.' 12.`That Self is hidden in all beings and does not shine forth, but it is seen by subtle seers through their sharp and subtle intellect.'

The words of verses 10-12 here are obscure.  But, as you know, the Great Self in sanskrit would be the Atman.  In this text the Undeveloped and the Person are beyond the Atman.  Here you can see early Hindu thinkers wrestling with how to understand the relation between various notions of what the Ultimate is like

1. The Hymns of the Rig Veda, translated with a popular commentary by Ralph T.H.Griffith, 1904. Reprinted in the Chowkhamba Sanskrit Studies, Vol XXXV, Varansi, India, 1971, 575-76.

2. [Griffith's note:] Warmth: Prof. Wilson, following Sayana, translates tapasah by "austerity," meaning the contemplation of the things that were to be created. M. Burnouf, La Sciences des Religions, pp. 207ff, has shown how warmth was regarded by the Aryas as the principle explaining movements, life, and thought.

3. Kama, eros, love.

4. ancient Rishis [i.e., "seers" or "prophets"].

5. Line: a line drawn by the ancient Rishis to make a division between the upper world and the lower, and to bring duality out of unity. [Compare to Plato's divided line?]

6. This translation is taken from The Upanishads, "translated by Max Müller," Part II, Clarendon Press, Oxford, 1884 (as in Dover reprint of 1962, 13-14).

7. "AUM": According to the Mandukya Upanishad, the "A" represents the conscious state; the "U" the state of dreaming; the "M" the state of dreamless sleep; and the silence that follows represents the dissolution of all individual self in the eternal and changeless Self.

8. In a famous 4th century B.C. writing, Plato compared the person's intellect to that of a charioteer driving two horses, the horse of concrete imagination and sensation, and the horse of passion. Whether this is coincidence is not known. Plato also adopted the idea of the transmigration of souls, a belief that appeared first in India 

9. This reference to Vishnu reflects a relatively new devotion to this god. Some of the Upanishads interpret the gods as manifestations of the supreme Self, just as humans are though in a lesser way than the gods.

10. Radhakrishnan and Moore, A Source Book in Indian Philosophy, Princeton Univ.Press, 1957, 42 (using the translation of R.E.Hume, The Thirteen Principle Upanishads, Oxford Univ.Pres, 1921.