Bhagavad-gita 2.29 - 2.30 Mystery of soul revealed through Sankhya yoga

Arjuna was affected by grief at death of his relatives and fear of sinful reactions. Since the soul who dwells in the body can never be slain, there was no need to grieve for any living being. Also since the instruction to fight was coming straight from the scriptures and spiritual master there was no fear of fall down.

Lord Krsna insisted on the Vedic conclusion that the living entity is the soul and not the body. Material bodies are perishable in due course of time  (antavanta ime deha) and soul is eternal and unchangeable (nityasyoktah saririnah). 

Example - The body is like a dream in which we think of flying in the sky or sitting on a chariot as a king. When we wake up we can see that we are neither in the sky nor seated on the chariot. Similarly the material body has no factual existence in relation to the eternal soul. 

The soul is amazing and the wonders of the individual soul are not easy to understand by mundane people. However the fact is that the atomic soul is within the body of every living entity be it human, animals, trees or insects as explained by the greatest authorities of knowledge.

Some people even remain misguided about the Supersoul and individual soul and out of ignorance do not accept the distinction of magnitude between them. Each of them fail in their ultimate understanding of the self.

Under the authority of a bona fide spiritual master and the causeless mercy of the pure devotees, the truth of Sankhya yoga or the science of self realization is revealed and one's life becomes successful.