Bhagavad Gita Verse 17, Chapter 18

yasya naahamkrito bhaavo buddhiryasya na lipyate |
hatvaapi sa imaanllokanna hanti na nibadhyate || 17 ||
 
One who does not have the notion that I am the doer, whose intellect is not tainted, he does not kill, nor is he bound, even by killing these beings.
 
yasya : one who
na : not
ahamkritaha : I am the doer
bhaavaha : notion
buddhihi : intellect
yasya : whose
na : not
lipyate : tainted
hatvaa : killing
api : even
saha : that person
imaan : these
lokanna : beings
hanti : kill
na : not
nibadhyate : bound
 
Previously, we came across the incorrect understanding of action. Whenever we think "I am performing this action", it is incorrect, born out of ignorance, it is durmati. Shri Krishna now gives us the correct understanding. When we think that "this action is being performed by the five factors of Prakriti, not by me", this is the correct understanding, this is sumati. And after the action is performed, we do not let the result of the action impact us. In other words, we are not attached to the reward of action.
 
The most common concern towards this kind of understanding is that it will make us weak and fatalistic, especially when we are still engaged in karma yoga. That is why it has to be combined with the idea of selfless service. A modern interpretation of this notion is : do your best, and leave the rest. As we continue our journey in karma yoga, our selfish desires and vaasanaas will slowly wither away, paving the way for the knowledge of the self in the second chapter to take root in our mind. Then we will come to the realization that only the self, the aatmaa is real or sat. Actions are in the realm of Prakriti, of Maaya, which is asat or illusory.
 
Per Shri Shankarachaarya’s commentary, this shloka embodies the sum and substance of the Gita and even of all the Vedas. We start our lives thinking that we are the body. The scriptures, the Vedas, tell us that we are not the body, we are the jeeva who has to use his body and mind to perform selfless service. Now, at the conclusion of the Vedas, Veda-anta, we are told that we are beyond the jeeva. This attitude of non-identification with the doership of actions differentiates a sanyaasi, a monk, from a tyaagi, one who has renounced action, per the original question of Arjuna in this chapter.
 
With the words "he kills, but does not kill", we are transported back to the second chapter, where Shri Krishna was convincing Arjuna to engage in the Mahaabhaarata war. So then, what is left? From a practical standpoint, we still have to deal with science of action. Unless we understand it fully, we will never be able to distance ourselves from the notion of doership. Shri Krishna continues his analysis of action from the standpoint of the three gunaas, since he has proven that action is in the realm of Prakriti.