Krishna uses a simple, observable biological fact to prove a profound metaphysical truth. He points out that the physical body is already constantly dying and being reborn. The body of the 'boy' (kaumāraṃ) dies to give birth to the 'youth' (yauvanaṃ), which dies to give birth to 'old age' (jarā). Since the 'I' (the observer) survives these massive physical transformations during a single lifetime, it will logically survive the final transformation of acquiring a new body (dehāntara-prāptir).