Highest Upanishad - Mandukya (AUM)

The Upanishad represent the philosophical cream of the Vedas; and of the 10 Upanishads the Mandukya Upanishad represents the highest. It contains 12 verses that explains the essence of AUM and everything is contained in its meaning. I have attached Sri Aurobindos' translation but there are many good links available (ie. www.swamij.com) and worth looking into if one is interested in the technical side of Vedanta.

THE MANDUKYA UPANISHAD
(translated by Sri Aurobindo)

1. OM is this imperishable Word. OM is the Universe, and this is the exposition of OM. The past, the present and the future, all that was, all that is, all that will be, is OM. Likewise all else that may exist beyond the bounds of Time, that too is OM.

2. All this Universe is the Eternal Brahman, this Self is the Eternal, and the Self is fourfold.

3. He whose place is the wakefullness, who is wise of the outward, who hath seven limbs, to whom there are ninteen doors, who feeleth and enjoyeth gross objects, Vaishwanara, the Universal Male. He is the first.

4. He whose place is the dream, who is wise of the inward, who hath seven limbs,to whom there are nineteen doors, who feeleth and enjoyeth subtle objects, Taijasa, the Inhabitant in Luminous Mind. he is the second.

5. When one sleepeth and yearneth not with any desire, nor seeth any dream, that is the perfect slumber, who is become Oneness, who is wisdom gathered into itself, who is made of mere delight , who enjoyeth delight unrelated, to whom conscious mind is the door, Prajna, the Lord of Wisdom, He is the third.

6. This is the Almighty, this is the Omniscient, this is the Inner Soul, this is the Womb of the Universe, this is the Birth and Destruction of creatures.

7. He who is neither inward-wise, nor outward-wise, nor both inward- and outward-wise, nor wisdom self-gathered, nor possessed of wisdom, nor unpossessed of wisdom, He Who is unseen and incommunicable, unseizable, featureless, unthinkable, and unnameable, Whose essentiality is awareness of the Self in its single existence, in Whom all phenomena dissolve, Who is Calm, Who is Good, Who is the One than Whom there is no other, Him they deem the fourth; He is the Self, He is the object of Knowledge.

8. Now this the Self, as to the imperishable Word, is OM : and as to the letters, His parts are the letters and the letters are His parts, namely, A U M.

9.The Waker, Vaishwanara, the Universal Male, He is A, the first letter, because of Initiality and Pervasiveness : he that knoweth Him for such pervadeth and attaineth all his desires; he becometh the source and first.

10. The Dreamer, Taijasa, the Inhabitant in Luminous Mind, He is U, the second letter, because of Advance and Centrality : He that knoweth Him for such, advanceth the bounds of his knowledge and riseth above difference : nor of his seed is any born that knoweth not the Eternal.

11. The Sleeper, Prajna, the Lord of Wisdom, He is M, the third letter, because of Measure
and Finality : he that knoweth Him for such measureth with himself the Universe and becometh the departure into the Eternal.

12. Letterless is the fourth, the Incommunicable, the end of phenomena, the Good, the One than Whom there is no other : thus is OM. He that knoweth is the Self and entereth by his self into the Self, he that knoweth, he that knoweth.