The In-dweller moves on from one body to another, when the former Deha collapses finally and dies. Death is for the Deha, not for the Dehi. For the Deha, all sensations that happen are also fleeting and impermanent. Sukha, Duhkha, heat , cold etc all come and go. They happen to Deha but are experienced by Dehi.
The wise man knows their fleeting nature and remains unperturbed by the changes happening in the body. Such a wise man who remains unperturbed by all these changes , is fit for immortality, says Lord Krishna. This will need further explanation. Let us move on with Lord Krishna further.
Vubhayorapi drastontastvanayostatvadarsibhih ||
The words 'sath' and 'asath' have special meanings in Vedanta. There are no equivalent words in English.
How do we know of anything which never existed, not existing now and will never exist in future at any time? How can we even talk of any such thing at all?
This is not the meaning in which the word asath is used here. Certain things give us the impression of being in existence continuously, but are not in such continuous existence. Consider the human body. When you look at your own body, you feel it is existing. But, you do know that, over a period of 10 years, either it has grown much larger or has depreciated much depending upon the stage of life you are in.
You also know well that this change is happening every moment in the body in every cell. One day, the quantum of change is so much that we are able to see it well with our gross eyes also. In reality, change is happening all the time, in every atom and every cell. You can't see the same body twice. You can't step into the same river twice, because water is flowing always though river looks like the same. Same is the case with your body.
If everything is MITHYA or ASAT in this universe, what then is SATH? What is it that remains UNCHANGED all the time? If something is UNCHANGED all the time, it must have been this way, always in the past, always in the present and always in the future. It can't disappear at any time in future, for, if it does that, it cannot be SATH. Likewise, it could not have not existed at any time in the past, for, then also, it could not have been SATH then.
Readers will remember the word 'DEHI' used by Krishna in previous slokas. Dehi was said to experience all the changes occurring in the Deha. In this sloka, the word used is Drasta, the witness or the Seer. The Drasta knows both the Sath and the Asath, says Lord Krishna.
But, is the Drasta Sath or Asath?
Vinaasamavyayasyaasya na kaschithkarthumarhasi ||
Sath is that which pervades all the asath - and yet is not asath. Asath is all that is pervaded by the Sath. The sath is indestructible; while the asath is continuously changing; or, in other words, it is continuously destroyed and formed into a different shape.
Not only the individual Deha which is asath and the Dehi, which is sath, the whole Universe is Asath and there is a Sath that pervades the whole Universe.
Anaasinoprameyasya tasmaad yudhyasva Bhaaratha ||
Having said this, Lord Krishna once again tells Arjuna, to fight the battle - since, whether you fight and destroy them or not, they are anyway bound for their own destruction.
The whole of Vedanta explores the Sath and Asath that fills the Universe, and explains to us the same. It calls the Sath as Brahmam and the Asath as Jagath, and gives their truth in a simple form as : "Brahma satyam; Jaganmithya". There is also a third aspect of this truth, to which we will come later.
In each Body and in every atom of the Universe, there is sath, which is permanent and indestructible. But, the Deha and the larger Universe, in fact, every atom of them are Asath and are constantly changing and getting destroyed.
So, the question now is - Are you the Deha , or, are you the in-dweller? Are you Asath, or , are you Sath?
This is the question that every Vedantin, every Yogi, every seeker of truth wants to explore and know. The Vedas and the Upanishads are all devoted to the unravelling of this one truth. Lord Krishna is giving their essence in the most easily understandable form in the Gita. We are proceeding along with the Lord to understand the same.