The Self /Atman सत् चित् आनन्दम् 

You must have understood that you have three bodies- gross, subtle and causal and these three bodies are nothing but the five sheaths- food sheath, vital air sheath, mental sheath, intellectual sheath and Bliss sheath. And you have also learnt that these five sheaths are not you but you own these.( like owner owns the house ) You have been given  this body mind complex (BMI) as per your vasanas ( in the previous births) to exhaust it in current birth, which is stored as the vasana layer of the causal sheath. Isn’t it?

Then who are you?

You are the Self/ Atman,  Which is covered / concealed by the five coverings/ sheaths.

Identifying with these sheaths now and then, you go on playing and experiencing as a waker, dreamer and the deep sleeper.  But you are not those characters, you are just the witness of all.
Now, we will learn about the Self/ Atman ( our True nature) in details.

If you need to tell someone about your house, how do you tell? You indicate it in two ways- You tell it with respect to surrounding things e.g. my house is that which has a saffron flag at the top , or by the side of which a big neem tree is there, etc. Another way is, you describe it with its qualities.  e.g A big three storyed green building, or small yellow colored house with conelike top , etc.

These descriptions surely give us idea about the house.

The nature of the Self also can be indicated in these two ways.
1. Tatastha lakshan/ततस्थ लक्षण- Self is indicated with respect to the world and its conditionings as the cause of the world. The witness of the thoughts and so on.

2.svaròop lakshan/ स्वरूप लक्षण- The Self is indicated directly as the nature of -Sat- Chit- Anand. (सत् चित् आनन्दम्)

1. Tatastha lakshan is when Self is indicated as the cause of the world.  Not only the creation of the world but its sustainance and destruction… Everything emerges from the Self and Everything get again mearged back in the Self/ Atman. Remember the deep sleep state. There is only the homogeneous causal layer, approximate to the Self. There is no world experience.
The Self  Atman is the witness of all the three states. In fact, Self is the substratum upon which all the three states come and go. Just like a white screen is the witness of all the events, characters of the movie getting played on it. It remains unaffected but without which movie cannot be played. After waking we again start facing, interacting  with the world of things and beings which is  created by us only to fulfill our desires. Every one has ones own created world.  In dream state also, mind creates its own world with the impressions of the waking state.
When we go to deep sleep,  there is no world but the complete Bliss. But after few hours, this Bliss also ends as sleeper wakes up as the same ignorant waker( does not know the source of the Bliss). 

You will seek the happiness in the outer world of things and beings only when you are ignorant of the very source of happiness, that is you…Self/ Atman.)  The realised masters who has experienced the Self, never seek it in the outside world. In fact, they never create their own world once realised.  They live in the world for the sake of others till the end of their prarabdha. 
It is we, ignorant of the Self, who has n number of vasanas to get the happiness and go on creating and expanding our own world.  Thus what is the root cause of our world?…. It is the Self( ignorance of the Self nature)

Let’s simplify this with an analogy. 

A lady is wearing the precious necklace,  but she forgets that she wore it. She thinks that the necklace is missing so she starts searching for it here and there. Asking everyone,  reacting, with anxiety, fear and anger. She continues her search till someone tells her that necklace is around her neck only. The moment she realises the fact, her all efforts end. She no more creates a drama(world)
Cause of her running here and there was the necklace. 

This is how you have to understand.  Eternal source of happiness is your Self. But because of this ignorance,  we are running after this and that creating and expanding the world.

Thus Self is that, which is the cause of the world.

2.  Now second way to indicate the Self is to know it’s nature/स्वरूप-i.e. Sat-chit-Anandam. सत् (Existence/Truth/Real), चित्(Consciousness/ knowledge), आनन्द(Bliss/ Eternal happiness)
These three characteristics sat, Chit and Ananda are not distinct from one another. Sat is present wherever there are Chit and Ananda. Atman/ Self is partless and homogeneous. Sat Chit Ananda is only a provisional definition of Atman. These are coeval and coexistent with Atman. You cannot split up sat Chit Ananda into three separate entities, just as you cannot separate light, heat and luminosity from fire. It is characterized for the purpose of easy understanding by us.

Sat/ सत्/Real/Truth/Existence-

Existence is that which remains unchanged in the three periods of time. (Past, present, future). The Self is present in all the three periods of time. It is opposite to asat/ unreal. Asat or unreal means false or mithya. Here we have to understand these term in proper way. Sat/सत is real that means unchanging or permanent, exists in all the three periods of time(Existence principle). Asat or unreal means that which is changing( temporary or ephemeral) not permanent.  Other than Self, everything changes that’s why everything which is not-Self is asat/ unreal.
In childhood,  in youth hood, now in adulthood,  your gross body , mind, intellect  has changed a lot. It will change in old age too. But do you notice, one constant entity is there in all these stages? That unchanging entity is the Self/ Atman,  the Real you. That’s why when you see your childhood photos, you say- ” I am that’ you remember that you were there. You as a Self was there, are here, will be there, even after the body you have been given dies.
Everything other than Self is the not-Self. ( Three bodies, three states, the world created of things and beings). Not-Self has beginning and end/death.  Birth is the coming into existence of a particular name and form. Death is their destruction. 

Self exists forever, what changes or dies is the not self. Even after bodies death, Self is as it is.

Ignorance of this Reality/ Self nature is the only cause of next birth with different name and form.

We wear different types of cloths. After the wear and tear, we discard it and wear a new one. We remain as it is.  Thus Real I/ our own Self is birthless & deathless, what changes is its outer coverings( Three bodies)
Everything exists because of this Existence Principle/ Sat aspect of the Self. It is not limited by time, place and person.   It is the substratum of the entire universe. It is experienced as the ‘ isness’ of the objects in the world. Without this existence Principle, neither objects nor the beings would be experienced. “The book is”, ” The table is.”, ” The tree is” This ‘ isness” of the object is existence principle/ Sat. In the same way,  “I am” here, “amness/isness of the I is the Existence principle.
Behind every object, beings, events in the movie is the screen.  We see only the changing objects, beings, events,  emotions,  etc. But their issness is because of the screen which does not change but gives the experience of its presence through the objects, etc. This is all about the Sat/ Existence aspect of the Self.

All about the Chit/ Consciousness & Ananda/ Bliss aspects, in next part.

           Hari Om!

Reference: Tatvabodh by Bhagvan Adi Shankaracharya., commentary by CIF.