Sankhaya Yog-Bhagavad Gita Chapter 2 [Contents of the Gita Summarized]
Krishna Said > Shaloka: 13
As the embodied soul continuously passes, in this body, from boyhood to youth to old age, the soul similarly passes into another body at death. A self-realized soul is not bewildered by such a change.
Since every living entity is an individual soul, each is changing his body every moment, manifesting sometimes as a child, sometimes as a youth, and sometimes as an old man. Yet the same spirit soul is there and does not undergo any change. This individual soul finally changes the body at death and transmigrates to another body; and since it is sure to have another body in the next birth-either material or spiritual-there was no cause for lamentation by Arjuna on account of death, neither for Bhisma nor for Drona, for whom he was so much concerned. Rather, he should rejoice for their changing bodies from old to new ones, thereby rejuvenating their energy. Such changes of body account for varieties of enjoyment or suffering, according to one’s work in life. So Bhisma and Drona, being noble souls, were surely going to have either spiritual bodies in the next life, or at least life in heavenly bodies for superior enjoyment of material existence. So, in either case, there was no cause of lamentation.
Any man who has perfect knowledge of the constitution of the individual soul, the Supersoul, and nature-both material and spiritual-is called a dhira or a most sober man. Such a man is never deluded by the change of bodies. The Mayavadi theory of oneness of the spirit soul cannot be entertained on the ground that the spirit soul cannot be cut into pieces as a fragmental portion. Such cutting into different individual souls would make the Supreme cleavable or changeable, against the principle of the Supreme Soul being unchangeable.