The 17th sloka dealt with the reasoning that since there is not a smaller entity than the atman, it is indestructible. In the 18th sloka, since the body was a composite of the five elements and since it was growing and waning, it is destroyable. In the 19th sloka, it was told that he, who does not understand this true nature of atman and body, is not a wise man. Having stated so, now in the 20th sloka, Sri Krishna establishes that atman does not undergo any changes whatsoever.
na jayate mriyate va kadacin
nayam bhutva bhavita va na bhuyah
ajo nityah sasvato ’yam purano
na hanyate hanyamane sarire
"For the soul there is neither birth nor death at any time. He was not born at the time of kalpa, does not disappear at the end of kalpa. He is unborn, eternal, unchanging and very ancient. He is not slain when the body is slain."
Anything that is eternal is unchanging and anything that is changeless is everlasting. All materials we see like flower and mud pots change as the time passes and so get destroyed. Earlier when He said atman is not born or die, Arjuna had a doubt that atman might not have been born now, but at the beginning of some centuries ago he could have been born. As a reply to that, Sri Krishna says kadacin- at anytime the atman was neither born nor will die at anytime. Because birth and death are related to the body. If atman and body were one, as the body changes from youth to old age, atman also should have been different. In reality, the same atman is in the changed body and so atman never undergoes any change. Human age is estimated to be about 100 years. Six months period from mid January – beginning of Tamil month Thai- is Utharayanam and the balance six months from mid July [corresponding to Tamil month Adi beginning] is Dakshinayanam. To the heavenly gods –Devas- each six-month period is a day or night. Thus our one-year is their one-day. So their age is 365X100 years of earthly human beings. They die at the end of this period. Even here also, the atman never dies. The god for creation, the four-faced Brahma, has much longer life. There are four yugas- Kritha, Thretha, Dwapara and Kali. The total period of the four yugas – one chatur yuga- is 4,320,000 years on earth. If 1000 chatur yugas pass it is one-day time for Brahma. So, one can calculate the Brahma’s life span! Even this atman of Brahma does not die or born. So the atman is eternal like the Paramatma. Such is the glorious nature of our souls. This does not mean our souls and Paramatma are same. If we develop our wisdom or gyana then we can have the knowledge of the Paramatma and also have the same degree of happiness. The great deluge [pralayam] is called kalpa. At the start of the kalpa, Brahma creates everything in this Universe. At that time also the atman is not created. Nor the atmans perish at the end of a kalpa. When a child is born, at that time an atman chooses that body and is born. Atman is there in everything. It is in this plant and in that pillar. One may wonder how atman can be in those inanimate objects. But if we examine closely, even our body is inanimate and we know atman is there. So, there is atman in all materials also. The only difference is that the atman in our body can develop his knowledge while, the atman in the pillar has no scope for wisdom. The atman in that pillar is there to expend the karma he had acquired and the moment that is over, that atman will depart that pillar and reside in another body. The magnitude of the wisdom of the atman in any body is determined by its past karma. If the wisdom is, let us say, 0%, it resides in an inanimate body like pillar. A slightly higher intellectual makes the soul to take a bird’s body or animal’s body. More wisdom makes the soul to take bodies of human beings or devas or even a Brahma. So the ranking of the knowledge decides the body that soul will be residing. The up and down in gyana or wisdom is only found in atmans. Once, the soul has taken a body, he tries to develop his wisdom and seek a higher level and ultimately, resides in the Sri Vaikuntam or His abode from where there is no more return for the atman to lower planes like this earth. Thus atman is eternal and never undergoes any change. Sri Krishna calls atman as purana. Pura api navaha puranaha is the Sanskrit meaning for the word purana. It means to say that it is very ancient and at the same time fresh or novel. Atman also is ancient but appears fresh everyday. It is common knowledge that we seek changes so as to avoid monotonous, which can be boring and dull. We rearrange our surroundings. We change our dress. So Arjuna feels that an unchanging and static atman might cause boredom. Sri Krishna allays such thoughts and says that atman is fresh everyday with more and better knowledge. This is a greatness of atman that though he is changeless, he appears afresh every time. Like Sri Aravamudan of Thirukkudanthai [Kumbakonam], Alwar says, every time he visits Him, he finds Him revealing Himself differently. We listen to Sri Ramayana. But every time we listen it refreshes our knowledge. So He now tells Arjuna that by killing Bheeshma, he will not kill the atman of Bheeshma.