Bhagavad Gita Verse 18, Chapter 2

antavanta ime dehaa nityasyoktaaha shareerinaha |
anaashinoprameyasya tasmaadyudhyasva bhaarata || 18 ||
 
These bodies of the eternal body-dweller will perish, it is said. It is imperishable and unfathomable. Therefore, fight, O Bhaarata.
 
antavanta : perishable
ime : these
dehaa : bodies
nityasya : eternal
uktaaha : has been said
shareerinaha: this body-dweller
anaashinaha : imperishable
aprameyasya : unfathomable
tasmaat : therefore
yudhyasva : fight
bhaarata : O Bhaarata
 
Like the previous shloka did, this shloka introduces another aspect of the eternal essence. It is unfathomable, incomprehensible. In other words, it cannot be understood by our intellect like we understand other kinds of knowledge. Also notice here that the eternal essence is denoted as one entity, whereas the bodies are many. Which means it is same eternal essence that pervades all material objects, including human bodies.
 
Since Shri Krishna concludes a line of reasoning in this shloka with the word "tasmaat", let's summarize the argument that began in verse 11 of this chapter:
 
1) Shri Krishna told Arjuna : "You are thinking that it is evil, wrong, unlawful to fight against your kinsmen. Your logic is incorrect. You are missing the big picture."
 2) "I will tell you the correct logic. You shouldn't grieve for them. The eternal essence, body dweller, is imperishable and real, whereas human bodies and material objects are perishable, and are unreal, as it were."
 3) "I will also give you some practical advice. Do not get agitated by joy and sorrow caused by contact with people, objects and situations. These are temporary conditions so bear them patiently. Once you learn to remain stable through joy and sorrow, you will begin to realize the eternal essence".
4) "Now that you know that you can never destroy the imperishable eternal essence, and that you should not grieve for the perishable, get up and fight, O Arjuna".
 
So then, what is the bottomline? "Arjuna, you should not think that you are this human body. Associate yourself with that body-dweller, that eternal essence. It will never get destroyed, so there is no need for grief."
 
Here we also see that Shri Krishna, like any good teacher, is providing 2 kinds of training : theoretical and practical. From the next shloka onwards, we will examine the theoretical, or logical aspect. In the later part of the chapter we will look at the practical aspect.
 
Footnotes
1. This shloka gives a pointer to the birth of the ego. When the eternal essence, which is one entity, associates itself with material objects, it gets split or fragmented. It begins to think that it is limited by whatever material object it is associated with. And when that eternal essence feels limited to a particular human body, and does everything in its power to maintain a sense of separation from everything else, that gives rise to the sense of ego.