As long as we identify ourselves with an individual human being, an ego, a personality, we are bound by the limitations of the specific form, and thus subject to death, disease, incapacity, pain and suffering. The Gita suggests that if we can shift our awareness and standpoint to that of the Eternal Brahman, our true existence, and see the form as part of the ever-changing play of Nature, without being caught up in identification with the specific form, we then experience existence from the standpoint of immortality.
If we observe a river, we can see that the river can be recognized separate from identification with any specific quantum of water that makes up that river at this particular moment. The specific droplets of water have passed on, but the river remains. This is a frequently cited general analogy for the kind of consciousness-shift that we are looking at here.
The human form that we inhabit is like the drop of water that passes along the river and out to sea, while the river continues. The Eternal Brahman (without trying to limit the Brahman in any way) is like the river that has always new water passing through it but remains unchanged by the specific changes that the drops of water undergo.
Sri Aurobindo discusses the issue in more general terms: “In the light of that sun of knowledge the eye of knowledge opens in us and we live in that truth and no longer in this ignorance. Then we perceive that our limitation to our present mental and physical nature was an error of the darkness, then we are liberated from the law of the lower Prakriti, the law of the mind and body, then we attain to the supreme nature of the spirit. That splendid and lofty change is the last, the divine and infinite becoming, the putting off of mortal nature, the putting on of an immortal existence.”
Sri Aurobindo, Essays on the Gita, Second Series, Part II, Chapter 13, The Field and Its Knower, pp. 405-406