As the embodied soul (master of the body) has childhood, youth and old age occurring to the body, so too is his change over to another body. The calm and wise man is not disturbed or perplexed in that regard. (Bhagavad Gita, Chapter 2, Verse 13)
The soul is the reality, not the body. All these kings men for whose approaching death Arjuna mourns, have lived before, they will live again in the human body; for as soul passes physically through childhood and youth and age, so it passes on to the changing of the body.
The calm and wise mind, the thinker who looks upon life steadily and does not allow himself to be disturbed and blinded by his sensations and emotions, is not deceived by material appearances; he does not allow the clamour of his blood and his nerves and his heart to cloud his judgement or to contradict his knowledge.
He looks beyond the apparent facts of the life of the body and senses. He looks into to the real fact of his being and rises beyond the emotional and physical desires of the ignorant nature. He constantly meditates on the true and only aim of the human existence – Self realization.