Sankhya Yoga 2.26

 Verse 2.26

अथ चैनं नित्यजातं नित्यं वा मन्यसे मृतम् |

तथापि त्वं महाबाहो नैवं शोचितुमर्हसि || 26||

atha chainaṁ nitya-jātaṁ nityaṁ vā manyase mṛitam

tathāpi tvaṁ mahā-bāho naivaṁ śhochitum arhasi

atha—if, however; cha—and; enam—this soul; nitya-jātam—taking constant birth; nityam—always; vā—or; manyase—you think; mṛitam—dead; tathā api—even then; tvam—you; mahā-bāho—mighty-armed one, Arjun; na—not; evam—like this; śhochitum—grieve; arhasi—befitting

Those who root for charvaka religion say that there is no such thing as eternal. Krishna has taken their claim for argument’s sake and explains even if atma is taken upon as an entity that is being born all the time, there is no reason to be sad about it. After their death at the battlefield, Bhishma and Drona will be born again in different forms. If atma has the nature of dying, one cannot change the very nature of a thing. If a thing is destined to die, it will eventually die. Even with regards to the death of the physical bodies Bhishma and Drona one is not qualified to grief.