Gita - Chapter 2, Verse 15-20

yaṁ hi na vyathayanty-ete

puruṣhaṁ puruṣharṣhabha

sama-duḥkha-sukhaṁ dhīraṁ

so-mṛtatvāya kalpate 

Meaning of Bhagavad gita Chapter 2, Verse 15

Arjuna, the wise man to whom pain and pleasure are alike, and who is not tormented by these contacts, becomes eligible for immortality.

---

nāsato vidyate bhāvo

nābhāvo vidyate sataḥ

ubhayor-api dṛṣhṭo-ntas

tv-anayos tattva-darśhibhiḥ 

Meaning of Bhagavad gita Chapter 2, Verse 16

That which is unreal (the physical body) has no existence. That which is eternal (the soul) has no destruction. Philosophers were the ones who saw and knew. They accepted this fact only after knowing the nature of both.

---

avināśhi tu tad-viddhi

yena sarvam-idaṁ tatam

vināśham-avyayasyāsya

na kaśhcit-kartum-arhati 

Meaning of Bhagavad gita Chapter 2, Verse 17

Know that the essence that pervades the entire body is indestructible. No one can destroy that immortal soul.

---

antavanta ime dehā

nityasyoktāḥ śharīriṇaḥ

anāśhino-prameyasya

tasmād-yudhyasva bhārata 

Meaning of Bhagavad gita Chapter 2, Verse 18

This material body is perishable. The soul is eternal, infinite and indestructible. Therefore, Arjuna, fight.

---

ya enaṁ vetti hantāraṁ

yaśh-chainaṁ manyate hatam

ubhau tau na vijānīto

nāyaṁ hanti na hanyate 

Meaning of Bhagavad gita Chapter 2, Verse 19

He who thinks that the soul is killed or that someone is being killed is not wise. The soul neither kills nor dies.

---

na jāyate mriyate vā kadācin

nāyaṁ bhūtvā bhavitā vā na bhūyaḥ

ajo nityaḥ śāśhvato-yaṁ purāṇo

na hanyate hanyamāne śharīre 

Meaning of Bhagavad gita Chapter 2, Verse 20

The soul is never born nor dies. It is ageless, eternal, and ancient. It does not die when the body perishes.

---

Comment :

Krishna is explaining to Arjuna the duality principle of the soul and the body. The soul neither dies nor is born. Soul is considered to be eternal and the real essence of the body. Both are inter-related but still the soul never perishes even if the body does. It is indestructible and so ageless. 

We can consider the soul, the same as we consider an atom in science. As per Dalton's Atomic theory, an atom can neither be created nor destroyed. This is the same principle that governs the theory regarding a Soul. It can form part of a bigger reaction but still each one of them taken separately, remains intact. The body can be considered here as an aggregate mass consisting of a number of atoms and energies within. It can hence be destroyed or separated, but still the individual atoms remain as is. Not being an expert in Science, I cant comment on this much more. But this idea can be used to gain a basic understanding of the soul and body duality principle.