Mahavakyas

Every upanishad has at least one mahavakya. A mahavakya literally means great statement. These statements are great because of their ability to directly indicate the identity of the jeevatma and the paramatma and destroy avidya.

Some of the more famous mahavakyas are -

1. prajnanam bramhan - aitreya upanishad

This means that consciousness is brahman. The word brahman comes form the sanskrit root brh which means expansion or vastness. Brahman is used to indicate the infinite, all pervading, non dual reality. Nothing can exist without consciousness. In absence of consciousness there is no observer. In absence of the observer there can be no observed, since it would not be observed even if it existed. Therefore without consciousness nothing exists, which leads to the conclusion that everything exists in consciousness. Therefore consciousness is all pervading and infinite, that which is infinite is non-dual. Since there cannot be two non dual entities, we conclude that consciousness is brahman.

2. aham brahmasmi - brhadaranyaka upanishad

This means I am brahman. Earlier we have deduced that brahman is consciousness. Here it is clearly stated that I am brahman. Thus I am consciousness. This utterly shatters all false identifications in the form of I am the body, I am the mind, I am the intellect etc. It establishes that I am the consciousness behind all the experiences of the body mind and intellect, I am observer, the body, mind, intellect, senses etc are only my instruments. I am infinite, I am all pervading. If carefully examined this statement destroys all sorrow. Sorrow stems form wanting something, from not having something, a desire frustrated, an expectation let down. Now if I am the infinite non-dual brahman, then nothing exists other than me. Therefore where is the question of desire. I can only desire something other than me, I can only have expectations when there is something other than me to expect something from. Thus this statement removes all sorrows at the root, restablishing the identity of myself as brahman alone.

3. Sarvam Khalvidam brahma - chandogya upanishad

This is translated as everything is truly brahman. Previously we said I am brahman. Of course only one logical step away is the conclusion that everything else is also brahman. Thus all likes and dislikes, all prejudices, all pairs of opposites like pain and pleasure, love and hate, etc resolve themselves, as mere illusions caused by our own false identifications.

There are many other great mahavakyas, deep reflection on these leads to great insights and revelations into truth.

tat twam asi - that thou art
ishavasyam idam sarvam - the lord is in all this
ayamatma brahma - this spirit is brahman
soham - I am Him.

a beautiful verse of Sri Adishankaracharya comes to mind...

asamgoham asamgoham asamgoham punah punaha
satchidananda roopoham aham eva aham avyayaha

I am unattached, I am unattached, I am unattached for ever.
I am the form of existence, consciousness and bliss, I alone am, I am eternal.

(I found this at http://www.hipforums.com/newforums/showthread.php?t=16759 )