Bhagavad Gita Verse 19, Chapter 2

ya enam vetti hantaaram yachshainam manyate hatam |
ubhau tau na vijaanito naayam hanti na hanyate || 19 ||
 
A person who believes that it (the eternal essence) slays, and another who believes that it is slain, neither of these understands. It does not slay, nor is it slain.
 
yaha : that person who
enam : this (body dweller)
vetti : believes
hantaaram : slayer
yaha : that
cha : and
enam : this
manyate : believes
hatam: slain
ubhau : both
tau : those
na : do not
vijaanitaha : know
na ayam : this does not
hanti : slay
na hanyate : nor is slain
 
The "It" in this shloka and the following shlokas refers to the body-dweller. Here Shri Krishna addresses another concern that Arjuna had raised. Arjuna thought that that he will be held responsible for killing his kinsmen, which was something that was abhorrent to him. But Shri Krishna through this shloka advised him to use the logic or the perspective of the eternal essence. The eternal essence does not kill, nor does it die from someone else trying to kill it.
 
But how does this apply to us? We are not warriors, and it is rare that we will be put in the position of killing somebody. So there must be another interpretation.This shloka equates the act of slaying to performance of any action, and being slain to any change or modification. In other words, the eternal essence never performs any action, nor does it undergo any change or modification.
 
As an example, let's consider at the sun. Without the sun there will be no activity or life on this planet. There would be no plant life because plants use the sun's energy. There would be no animal or human life because both cannot survive without plants. But, does the sun perform any action pertaining to growing a plant or an animal? Does it get affected by all the changes happening on earth? It does not that that "I caused this forest to grow" or "I was impacted by this eclipse". It remains actionless and changeless.
 
The shloka goes on to say that one who thinks that the eternal essence acts or kills is not using viveka or discrimination, and that person does not have the correct understanding. The person is still thinking at the level of the material object, or in other words, the level of the un-real, and has still not learnt to discriminate between the two.
 
Footnotes
 
1. Verses 19 and 20 are almost verbatim taken from the Katha Upanishad.