Chapter2- Sloka 21
vedāvināśinaṃ nityaṃ ya enam ajam avyayam |
kathaṃ sa puruṣaḥ pārtha kaṃ ghātayati hanti kaṃ || 21 ||
(2.21)
pārtha- O son of Pṛthā (Arjuna)!; yaḥ (puruṣaḥ)- one who (the puruṣa who); veda- knows; enaṃ- this soul; avināśinaṃ- as one who is indestructible; ajam- as one who has no birth; avyayam- as one without any flaws (doṣas); nityaṃ- as one who exists at all times; kathaṃ saḥ puruṣaḥ- How can such a puruṣa; hanti- kill; kaṃ- any soul; kathaṃ vā- how can he; ghātayati kam- get any soul killed (by others);
Purport
In the earlier śloka 2.19 Kṛṣṇa explained that one who thinks that the soul can get killed is ignorant. In this śloka it is explained that one who knows the soul to be eternal cannot either kill the soul or get it killed.
Kṛṣṇa says to Arjuna, "O Pārtha! The puruṣa who knows this soul to be indestructible, unborn, unchanging and eternal, how can he kill any soul or get it killed by someone?"
yaḥ puruṣaḥ (That conscious being who...) One who is not aware of his own nature cannot be a conscious being.
avināśinaṃ veda (Knows as indestructible) With the word avināśinaṃ, it is said that the soul does not undergo the modification called 'vinaśyate (death)' since it is indestructible.
enaṃ (This soul) The word enaṃ refers to the nature of the soul as established in the previous śloka.
ajam veda (Knows (the soul) as unborn) With the word ajam, it is said that the soul does not undergo the modification called 'jāyate (birth)', since it is unborn.
avyayam veda (Knows (the soul) as flawless.) Growth and change can occur only to a substance having flaws. The term 'vyaya' refers to all the transformations that occur except birth and destruction. Therefore with the word 'avyayam', it is said that the soul does not undergo the other four modifications namely parinamate(undergoing change), vivardhate (growing or changing), apakṣīyate (dwindling), asti (is with changes). Alternatively if it is assumed that the word avyayam refers to only one modification namely apakṣīyate (dwindling), it can be understood that its mention implies the other three modifications too.
nityam veda (Knows 'the soul' to be always existing without any changes) With the word nityam, it is said that the soul is eternal and unchanging since it is not subject to the ṣaḍbhāvas (six transformations) namely jāyate( birth), parinamate( transformation), vivardhate (growth), apakṣīyate (dwindle), vinasyate (death), asti (remaining in each state).
saḥ puruṣaḥ (Such a conscious being...) With these words such a person's speciality of knowing all the above characteristics of the soul is conveyed.
katham hanti (How can he kill?)
kam (Any soul residing in either devata (celestial beings), manuṣya (human), tiryak (moving) or stāvara ( stationery) bodies)
katham ghātayati (How can he cause such a soul's killing by others?)
It is impossible for someone who knows that the soul cannot be killed by any weapon or any substance, to think of killing the soul or getting it killed by others.
Summary:
The śloka tells us that the soul is indestructible and free from the six transformations that the body undergoes. One who knows this cannot even imagine that he can kill the soul or get it killed in any way. Earlier Arjuna had expressed his concern that, "How can i kill the souls of grandsire Bhīṣma, guru Droṇa, Duryodhana and others? How can I get them killed by Bhīma and other powerful warriors? Kṛṣṇa's response to Arjuna is that, "Your concern and grief in this matter are due to your ignorance about the real nature of the soul, O Arjuna!".