Bhagavan Shri Krishna said: While speaking like a wise man, you are actually grieving for that which is unworthy of grief. The wise neither lament for the living nor the dead.
Bhagavad Gita - Chapter 2 - Sankhya Yoga - The Yoga of Analysis
There was never a time that you, nor I, nor all these warriors assembled here did not exist. Nor shall we ever cease to exist in the future.
As the atma passes through the bodily transformations of childhood, youth and old age, it similarly transmigrates from one body to another at the time of death. The wise are never deluded by this transition.
O son of Kunti, the interaction between the senses and the sense-objects produce the sensations of cold, heat, pleasure and pain. These feelings are temporary, always appearing and then disappearing. Thus, O descendent of Bharata, you must learn to tolerate them.
O most virtuous one, a sober man who is equipoised in both pleasure and pain and remains undisturbed is certainly qualified for liberation.
Of that which is temporary there is no eternal existence. Of that which is eternal there is no destruction or change. Seers of the truth have realized the constitutional position of both.
Know for certain that individual consciousness, which pervades the whole body, is imperishable. Nobody can destroy the indestructible individual unit of consciousness.
Embodied consciousness is eternal, imperishable and infinite. Only the material body is perishable. Therefore O Arjuna, fight!
One who considers the eternal unit of consciousness to be the slayer, and one who considers it to be capable of being slain are both in ignorance - for it neither slays nor is slain.
The individual unit of consciousness neither takes birth nor dies at any time. It has never been created nor will it ever be created. It is unborn, eternal, indestructible and timeless - it is not destroyed when the material body is destroyed.