Bhagavad Gita Verse 26, Chapter 2

atha chainam nityajaatam nityam vaa manyase mritam |
tathaapi tvam mahaabaaho naivam shochitumarhasi || 26 ||
 
But on the other hand, if you believe that this (eternal essence) constantly takes birth and dies, then also you should not grieve, O mighty armed.
 
atha cha : but if
enam : this (eternal essence)
nityajaatam : constantly being born
nityam : constantly
vaa : or
manyase : believe
mritam: die
tathaapi : then also
tvam : you
mahaabaaho : O mighty-armed
na : not
evam : this
shochitum : grieve
arhasi : should
 
Shri Krishna imparted the teaching of the eternal essence to Arjuna in the shlokas we've been seeing. Now, the teaching becomes slightly less abstract and a little more "down-to-earth" as it were. Shri Krishna understands that people like us may find it hard to comprehend the notion of imperishability. Our mind will find a hard time comprehending that something was never born/created, and something will never die/get destroyed.
 
For example, we may think of the Earth as relatively imperishable, but even then we know from our school geology class that it was formed billions of years ago out of cooling of gases.
 
Therefore, Shri Krishna says : "O Arjuna, even if you cannot comprehend that the eternal essence is imperishable, it is ok, you can also believe that the eternal essence, which is present in human bodies, undergoes birth and death with the body". In the next few shlokas, Sri Krishna will convince Arjuna that his grief is unfounded even if he accepts that the eternal essence undergoes birth and death.
 
We may have a question here: Why did Shri Krishna begin his teaching of the Gita by describing the eternal essence? Isn't it a complex, abstract topic that all of us, including Arjuna, would have a hard time understanding? Couldn't he have started with something simpler?
 
Here's a possible answer. What is described as the eternal essence is the goal, the final result of the Gita teaching. Like a good teacher, Shri Krishna described the end goal to his students before describing the means to attain that goal. Any good teacher will always describe, elaborate upon, and glorify the end goal in the first lecture of a course. This will generate interest, curiosity, focus and dedication from the student. The student may not necessarily understand everything about that goal, but that is ok.