Ekatama Darshana

Brahman is understood with two kinds of lakshanas.

Tatastha lakshana, and svarupa lakshana

Tatastha lakshana defines brahman as jagad kAranam. It is the vivarta upAdana karanam, with maya shakti. Maya is mithya, and trigunatmika. Maya is brahman. Brahman is satyam, triguna rahita, or nirguna.

Svarupa lakshana defines brahman to be satchitananda atma. Chit refers to self evident consciousness. We understand atma as the witness consciousness, that is self evident. Witness the name is with reference to the witnessed, so the witnessing is a nama rupa, and by itself atma is just chit, self evident. Hence atma or brahman need not be known , and need not be a knower also. It exists, is self evident, and self existent. Also it is anantam, as it does not have the limitations belonging to the known universe, such as space time limitation, or attribute wise limitations.

Hence atma is satchitananda, atma is brahman, satchitananda.

So in two fold manner brahman is understood. In terms of its svarupa lakshana, brahman is satyamjnanamanantam. 

In terms of tatastha lakshana, brahman is jagad karanam, wherein the karanam status is mithya. The karyam is non separate from karanam and is also mithya.

This two fold status is important as it establishes non duality as the truth. Advaitam as the truth, and oneself as the advaita satyam.

Oneself need not be known. Oneself need not be proved to be limitless or self existent. Oneself is to be understood to be brahman. Brahman is to be understood as satyam. Jagad or ishvara(bhagavan) being mithya is to be understood as none other than oneself, oneself is the being , the reality.

Tat Tvam Asi

   Tat 

     The vachyartha of tat, is the jagad karanam ishvara, who is bhagavan. Owner of six bhagas.

ऐश्वर्यस्य समग्रस्य वीर्यस्य यशसः श्रियः।

ज्ञान-वैराग्ययोश्चैव षण्णां भग इतीरणा॥

These are: all knowledge, jnana; total dispassion, vairagya; the capacity     to create, sustain, and resolve, virya; absolute fame,yasas; all        wealth, sri; and overlordship, aisvarya.

The lakshyartha, is satyam brahma. The reality of ishvara, which is satchitananda.

What is satchitananda brahman?

Tvam 

Tvam in general refers to jiva. But when we look at oneself atma, considered to be jiva, atma is 
    sakshi chaitanyam, chit, who is witness consciousness, of 3 states of waking dreaming sleeping,
   confused with the panchakoshas, and with the dehatrayam (sthula sukshma karana sharira), but
   sakshi chaitanyam is pure consciousness, and being witness to space and time, is limitless in terms
   of space and time, and self existent.

This sakshi chaitanyam is brahman, the jagad karanam.

This is mahavakya, wherein the bhagavan is equated with jiva as one and the same. That is
   understood by taking the lakshyartha. Lakshyartha refers to the one truth, which is non dual
   satchitananda brahman, who is alone atma.

Doership, ahamkara is sublated by this knowledge. It is a mere ornament for the survival of the
    body mind, which itself is ishvara srishti, wherein the total maya sharira is pervading the individual
    body mind as well.

So let there be no doubt with respect to the fact of atma being brahman, and deha and the
    universe being mithya karyam. The seeing doing is ishvara anugraha, and maya anugraha, whole
   jagad can be appreciated as an effect, and oneself understood to be non separate reality or
   satyam.

Unfolding I as brahman with QnA

Q: What does shastra say

A: Its says I or self am brahman

Q: What is brahman?

A: Brahman is cause of the universe all that is here.

Q: What is meant by all that is here?

A: All that is here means, from perspective of human being, it means all that he or she knows, all that he or she doesnt know, as well as himself the subject or the knower.

Q: So does it mean everything , the whole?

A: Yes it means every thing, the whole

Q: So shastra says the whole has only one cause?

A: Yes the whole has only one cause

Q: How can self be the cause, the self is just the body mind, limited I.

A: This conclusion is due to non examining. You are the sakshi, the knower of your body mind, emotions, and knower of sleep state , dream state, waking state. You are the consciousness.

Q: So I the consciousness is the cause of All?

A: Yes, cause of all, brahman

Q: But I find that I am still here, and I havent become all, I am not a bird, I am not a tree and so on. Also I am limited to being the seer of this mind alone.

A: A bird a tree IS brahman, while brahman is free from names and forms. Seer is with respect to the known, its a name as well. All of this is Brahman or consciousness .