Sankhya Yoga 2.18

Verse 2.18 Atma is eternal and never changing

अन्तवन्त इमे देहा नित्यस्योक्ता: शरीरिण: |

अनाशिनोऽप्रमेयस्य तस्माद्युध्यस्व भारत || 18||

antavanta ime dehā nityasyoktāḥ śharīriṇaḥ

anāśhino ’prameyasya tasmād yudhyasva bhārata

anta-vantaḥ—having an end; ime—these; dehāḥ—material bodies; nityasya—eternally; uktāḥ—are said; śharīriṇaḥ—one who has the body; atma; anāśhinaḥ—indestructible; aprameyasya—non objectifiable; tasmāt—therefore; yudhyasva—fight; bhārata—descendant of Bharat, Arjun

Only the material body is perishable; atma that has the body is indestructible, non objectifiable, and eternal. Therefore, fight, O descendent of Bharata.

Atma kutastha nityam; atma aprameyam

Nityam means eternal. The word kutastha specifies an important attribute of atma. There are two kinds of nityam, eternity. One is pravaha nityam - eternal but always flowing, changing; the other is kutastha nityam - eternal and never changing. Here it is proved that atma is kutastha nityam.

Sri Krishna also says atma aprameyam - atma is non objectifiable; it cannot be known.

This physical body is mitya

In one angle it can be considered that the human body is an assemblage of bones, flesh, skin, blood, cells and DNA. In another, we can say that the human body is made up of protein, carbohydrate, fat, minerals and water. Since all these things depend on each other to make up the whole body and the body itself depends on these things for its construction and function, the physical body is mitya. Even according to the vision of vedanta, since the physical body is dependent on the five elements, it is considered mitya. 

The physical body is a product. The body appears at a certain period in time, exists for a certain amount of time, and withers away after a period of time. In this verse, Krishna points out all the bodies (including that of his own) of those people who gathered in the battlefield and says they are of the nature of having an end (anthavantha:).

To whom do these bodies belong? It is the atma that lends existence and consciousness to the body. Since the consciousness of atma reflects in the subtle body (sukshma sarira), the physical body too attains consciousness. Therefore, the physical body is dependent on the atma that dwells in it.

When referring to the physical bodies Krishna uses a plural word ime deha: (these bodies), but when he wants to refer to the one who has these bodies, he uses a singular word saririna: (the one who has these bodies). Hence he wants to establish that though the bodies that have the nature of having an end are many, the atma that dwells inside all of them is only one, not many.

Baghvan here teaches us three truths in one single line. 1) Though there are many bodies, atma is only one. 2) Bodies will perish but atma is indestructible. 3) Bodies are products and therefore known as objects but atma is non objectifiable. 

Atma is self evident (atma swada siddha:) 

Shankara in his commentary says atma cannot be known as an object. If it cannot be known then what is the point of learning vedas and shastras (scriptures)? What role do they play in knowing the atma? The role of upanishad is removing all wrong notions about one’s self (atma) thereby revealing the self evident atma. Because of this important job, upanishads gain the status of a pramana (instrument of knowledge).  In reality, atma is swada siddha: self evident. But due to wrong identification, we superimpose the doership and the enjoyership on atma and suffer due to this. By nature, every jeeva holds this wrong conclusion in his heart. Upanishad only clears out this confusion. The ability of shastra to remove the ignorance that is superimposed on atma due to wrong understanding gives the shastra the status of an instrument of knowledge, pramana.

Shastra does not say that atma can be known. Shankara quotes the shastra that only because of the self-evident atma everything else becomes evident. In this verse Krishna says the bodies are destructible and the atma that has the bodies as its abode is indestructible. Why can't it be destructible? Because atma is the knower, not the known (aprameyam). All known objects are subject to destruction. Since atma is self evident, and not a known object, it is eternal. Since it cannot be measured by time it is indestructible. 

Moksha shastra ( the scripture for liberation)

Why does Krishna tell Arjuna to fight here? If Arjuna wants to destroy the atma of Bhishma he will not be able to do it since atma is indestructible. If he wants to prevent the destruction of Bhishma’s body he would not be able to do it either. Krishna here does not order him to fight. Arjuna had come to the battle ground just for the purpose of waging a war. Due to delusion and sorrow he did not know how to proceed with the things. Krishna simply removes his sorrow and delusion and asks him to do his duty. One should not escape from performing one’s duty. Since Arjuna’s duty is to fight, at this instance he asks him to fight. Because Krishna tells Arjuna to fight, we should not consider Gita as a pravritti shastra (the scripture that teaches the path of acquisition) like the karma kanda of vedas. Since it removes Arjuna’s sorrow and its root cause, ignorance, we should consider Gita as a nivritti shastra (the scripture that teaches the path of renunciation).

The benefit of performing one’s duty is attaining one’s mental purity. Through attaining mental purity one qualifies for Self knowledge (Atma jnana). Through attaining self knowledge one attains Moksha (liberation).